V4L/DVB: au8522: fix case where we don't perform the first tune after going digital

Address a problem found in MythTV where if we are in digital mode, switch to
analog mode, and the switch back to digital mode, the first tuning request
after switching back to digital mode gets dropped.  This is because the au8522
maintains internal state, and would think the demod was already tuned to the
target frequency.

Thanks to Zaphod Beeblebrox for reporting this issue.

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