[media] m920x: factor out a m920x_parse_rc_state() function

This is in preparation to using RC core infrastructure for some devices,
the RC button state parsing logic can be shared berween rc.legacy and
rc.core callbacks as it is independent from the mechanism used for RC
handling.

Signed-off-by: Antonio Ospite <ospite@studenti.unina.it>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>