[media] gspca: Support variable resolution

Add variable resolution support to gspca by allowing subdrivers to
specify try_fmt and enum_framesizes functions.

Signed-off-by: Ondrej Zary <linux@rainbow-software.org>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>