[media] m920x: factor out a m920x_write_seq() function

This is in preparation for the vp7049 frontend attach function which is
going to set a sequence of registers as well.

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