[media] radio-si4713: use V4L2 core lock

Simplify locking by using the V4L2 core lock mechanism. This allows us to
remove all locking from the i2c module. This will also simplify the upcoming
conversion to the control framework.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Eduardo Valentin <edubezval@gmail.com>
Tested-by: Eduardo Valentin <edubezval@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>