[media] cx231xx: Provide signal lock status in G_INPUT

Make use of the signal state registers to properly populate the signal lock
registers in the cx231xx driver.

This allows applications to know whether there is a signal present even in
devices which lack a tuner (since such apps typically won't call G_TUNER if
no tuner is present).

[mchehab@redhat.com: Fix CodingStyle: don't use {} for one-line if's]
Signed-off-by: Devin Heitmueller <dheitmueller@kernellabs.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>