[media] au8522: set signal field to 100% when signal present

The signal state field in G_TUNER is typically scaled from 0-100%.  Since we
don't know the signal level, we really would prefer the field to contain 100%
than 1/256, which in many utilities (such as v4l2-ctl) rounds to 0% even when
a signal is actually present.

This patch makes the behavior consistent with other drivers.

Signed-off-by: Devin Heitmueller <dheitmueller@kernellabs.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>