[media] si2157: detect if firmware is running

Detect if firmware is running run-time and download / start it only
when needed. Detection is done by reading IF frequency value.
Garbage value is returned by firmware when it is not running,
otherwise correct value is returned.

Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>