[media] fc0013 ver. 0.2: introduction of get_rf_strength function

Changes compared to version 0.1 of driver (sent 6 May):
- Initial implementation of get_rf_strength function.
- Introduction of a warning message

Signed-off-by: Hans-Frieder Vogt <hfvogt@gmx.net>
Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>