V4L/DVB (13173): adv7180: Support for getting input status

This patch adds support to the ADV7180 driver to check the input
status.

Since the status is held in the same register as the input standard
a small restructuring of the code is done to reuse the code for
reading the register

Signed-off-by: Richard Röjfors <richard.rojfors@mocean-labs.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>