[media] gspca: Fix underflow in vidioc_s_parm()

"n" is a user controlled integer.  The code here doesn't handle the case
where "n" is negative and this causes a static checker warning.

	drivers/media/usb/gspca/gspca.c:1571 vidioc_s_parm()
	warn: no lower bound on 'n'

parm.capture.readbuffers is unsigned, so make n unsigned too, fixing this.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>