V4L/DVB (11575): uvcvideo: fix uvc resume failed

Now urb buffers is not freed before suspend, so uvc_alloc_urb_buffers should
return packet counts allocated originally during uvc resume, instead of zero.

This version uses round down to return packet counts on Linus' suggestions,
or else may lead to buffer destructed if packet size is changed before
calling uvc_alloc_urb_buffers() in this kind of case.

Signed-off-by: Ming Lei <tom.leiming@gmail.com>
Acked-by: Laurent Pinchart <laurent.pinchart@skynet.be>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>