usb: dwc2: bail out early when booting with "nousb"

usb: dwc2: bail out early when booting with "nousb"

Add usb_disabled() check to prevent kernel oops when booting with "nousb"
in the cmdline:

Unable to handle kernel NULL pointer dereference at virtual address 00000030
...
PC is at bus_add_device+0xe0/0x18c
LR is at device_add_groups+0x1c/0x20
...
[<c02191c0>] (bus_add_device) from [<c0217130>] (device_add+0x41c/0x538)
[<c0217130>] (device_add) from [<c023b1d4>] (usb_new_device+0x270/0x35c)
[<c023b1d4>] (usb_new_device) from [<c0241174>] (usb_add_hcd+0x4fc/0x760)
[<c0241174>] (usb_add_hcd) from [<c0254ce0>] (dwc2_hcd_init+0x434/0x510)
[<c0254ce0>] (dwc2_hcd_init) from [<c02594f4>] (dwc2_driver_probe+0x130/0x170)
[<c02594f4>] (dwc2_driver_probe) from [<c021bbd0>] (platform_drv_probe+0x28/0x58)

Signed-off-by: Andre Heider <a.heider@gmail.com>
Acked-by: Paul Zimmerman <paulz@synopsys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>