usb: Fix bootindex for portnr > 9

usb: Fix bootindex for portnr > 9

We identify devices by their Open Firmware device paths.  The encoding
of the host controller and hub port numbers is incorrect:
usb_get_fw_dev_path() formats them in decimal, while SeaBIOS uses
hexadecimal.  When some port number > 9, SeaBIOS will miss the
bootindex (lucky case), or apply it to another device (unlucky case).

The relevant spec[*] agrees with SeaBIOS (and OVMF, for that matter).
Change %d to %x.

Bug can bite only with host controllers or hubs sporting more than ten
ports.  I'm not aware of any.

[*] Open Firmware Recommended Practice: Universal Serial Bus,
Version 1, Section 3.2.1 Device Node Address Representation
http://www.openfirmware.org/1275/bindings/usb/usb-1_0.ps

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Note: xhci can be configured with up to 15 ports (default is 4 ports).

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>