V4L/DVB (8000): tda827x: fix NULL pointer in tda827xa_lna_gain

Check that tda827x_config is defined before attempting to use it.

Signed-off-by: Sigmund Augdal <sigmund@snap.tv>
Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>