[media] em28xx: introduce #define for maximum supported scaling values (register 0x30-0x33)

The maximum supported scaling value for registers 0x30+0x31 (horizontal scaling)
and 0x32+0x33 (vertical scaling) is 0x3fff, which corresponds to 20% of the
input frame size.

Signed-off-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>