[media] si2157: avoid firmware loading if it has been loaded previously

Add a variable into state to keep track if firmware has been loaded or not.
Skip firmware loading in case it is already loaded (resume from sleep).

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