Check return value from call to af9013_wr_regs(), so in case of
error print debug message and return.

Addresses-Coverity-ID: 1227035
Signed-off-by: NGustavo A. R. Silva <garsilva@embeddedor.com>
Signed-off-by: NAntti Palosaari <crope@iki.fi>
Signed-off-by: NMauro Carvalho Chehab <mchehab@s-opensource.com>