[media] gspca: rename wxh_to_mode() to wxh_to_nearest_mode()

The name wxh_to_nearest_mode() reflects better what the function does.

Signed-off-by: Antonio Ospite <ao2@ao2.it>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>