[media] uvcvideo: Fix ENUMINPUT handling

Properly validate the user-supplied index against the number of inputs.
The code used the pin local variable instead of the index by mistake.

Reported-by: Jozef Vesely <vesely@gjh.sk>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: stable@vger.kernel.org
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>