[ALSA] sound/usb/usbaudio: Handle return value of usb_register()

Handle the return value of usb_register() in the module_init function.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>