greybus: spi: Allow spi-core to allocate bus numbers dynamically

greybus: spi: Allow spi-core to allocate bus numbers dynamically

And that fixes these warnings generated with BDB:

WARNING: at /home/viresh/ara/jetson-kernel-build/tegra/fs/sysfs/dir.c:530 sysfs_add_one+0xa4/0xb4()
sysfs: cannot create duplicate filename '/class/spi_master/spi0'
Modules linked in: gb_es2(O) gb_phy(O) greybus(O)
CPU: 2 PID: 111 Comm: kworker/u8:3 Tainted: G        W  O 3.10.40-gf32f9c5ca7e8 #2
Workqueue: events_unbound svc_process_hotplug [greybus]
[<c0016844>] (unwind_backtrace+0x0/0x13c) from [<c0012fc4>] (show_stack+0x18/0x1c)
[<c0012fc4>] (show_stack+0x18/0x1c) from [<c0067d4c>] (warn_slowpath_common+0x5c/0x74)
[<c0067d4c>] (warn_slowpath_common+0x5c/0x74) from [<c0067d9c>] (warn_slowpath_fmt+0x38/0x48)
[<c0067d9c>] (warn_slowpath_fmt+0x38/0x48) from [<c01bc218>] (sysfs_add_one+0xa4/0xb4)
[<c01bc218>] (sysfs_add_one+0xa4/0xb4) from [<c01bcc24>] (sysfs_do_create_link_sd+0xc0/0x20c)
[<c01bcc24>] (sysfs_do_create_link_sd+0xc0/0x20c) from [<c042e088>] (device_add+0x2e8/0x5f4)
[<c042e088>] (device_add+0x2e8/0x5f4) from [<c04ae008>] (spi_register_master+0x15c/0x654)
[<c04ae008>] (spi_register_master+0x15c/0x654) from [<bf015df4>] (gb_spi_connection_init+0x164/0x19c [gb_phy])
[<bf015df4>] (gb_spi_connection_init+0x164/0x19c [gb_phy]) from [<bf002604>] (gb_connection_bind_protocol+0x160/0x1b4 [greybus])
[<bf002604>] (gb_connection_bind_protocol+0x160/0x1b4 [greybus]) from [<bf002880>] (gb_connection_create_range+0x228/0x2fc [greybus])
[<bf002880>] (gb_connection_create_range+0x228/0x2fc [greybus]) from [<bf002994>] (gb_connection_create+0x40/0x48 [greybus])
[<bf002994>] (gb_connection_create+0x40/0x48 [greybus]) from [<bf000be0>] (gb_manifest_parse+0x61c/0x628 [greybus])
[<bf000be0>] (gb_manifest_parse+0x61c/0x628 [greybus]) from [<bf0019ac>] (gb_interface_init+0x130/0x170 [greybus])
[<bf0019ac>] (gb_interface_init+0x130/0x170 [greybus]) from [<bf003bf0>] (svc_process_hotplug+0x214/0x258 [greybus])
[<bf003bf0>] (svc_process_hotplug+0x214/0x258 [greybus]) from [<c0087ecc>] (process_one_work+0x13c/0x454)
[<c0087ecc>] (process_one_work+0x13c/0x454) from [<c0088c20>] (worker_thread+0x140/0x3dc)
[<c0088c20>] (worker_thread+0x140/0x3dc) from [<c008f20c>] (kthread+0xe0/0xe4)
[<c008f20c>] (kthread+0xe0/0xe4) from [<c000f098>] (ret_from_fork+0x14/0x20)

Reported-by: Mitchell Tasman <tasman@leaflabs.com>
Suggested-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>