[media] af9033: fix compiler warnings

Fix two warnings:

af9033.c: In function 'af9033_read_status':
af9033.c:883:25: warning: 'snr_lut' may be used uninitialized in this function [-Wmaybe-uninitialized]
   const struct val_snr *snr_lut;
                         ^
af9033.c:952:25: warning: 'tmp' may be used uninitialized in this function [-Wmaybe-uninitialized]
   c->cnr.stat[0].svalue = tmp;
                         ^

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>