[media] rtl28xxu: return success for unimplemented FE callback

Return success for FE callback on case we don't have any special
implementation. fc0013 tuner driver calls that callback in order to
switch antenna input, even we don't provide antenna switch.
Returning error caused fc0013 driver given up tuning.

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