V4L/DVB (6622): tda827x: prevent possible NULL pointer dereference in tda827xa_lna_gain

If tda827x_config hasn't been defined, exit the function.

Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>