[media] pwc-if: must check vb2_queue_init() success

drivers/media/usb/pwc/pwc-if.c: In function 'usb_pwc_probe':
drivers/media/usb/pwc/pwc-if.c:1003:16: warning: ignoring return value of 'vb2_queue_init', declared with attribute warn_unused_result [-Wunused-result]
In the past, it used to have a logic there at queue init that would
BUG() on errors. This logic got removed. Drivers are now required
to explicitly handle the queue initialization errors, or very bad
things may happen.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>