V4L/DVB: V4L2: Replace loops for finding max buffers in VIDIOC_REQBUFS callbacks

Due to obvious copy and paste coding a number of video capture drivers
which implement a limit on the buffer memory decremented the user
supplied buffer count in a while loop until it reaches an acceptable
value.

This is a silly thing to do when the maximum value can be directly
computed.

Signed-off-by: Andreas Bombe <aeb@debian.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>