[media] au0828: improve firmware loading & locking

- open/close/read and poll need to take the core lock as well.
- when the tuner goes to sleep we should set std_set_in_tuner_core
  to 0 since the tuner loses the firmware at that time.
- initialize the tuner if std_set_in_tuner_core == 0 whenever:
  1) g/s_tuner, s_std or s_frequency is called
  2) read or poll is called
  3) streamon is called

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Reviewed-by: Devin Heitmueller <dheitmueller@kernellabs.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>