[media] pwc: hide unused label

The pwc driver causes a warning when CONFIG_USB_PWC_INPUT_EVDEV is unset:

drivers/media/usb/pwc/pwc-if.c: In function 'usb_pwc_probe':
drivers/media/usb/pwc/pwc-if.c:1115:1: warning: label 'err_video_unreg' defined but not used [-Wunused-label]

This moves the unused label and code inside another #ifdef to
get rid of the warning.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>