media: cec-funcs.h: cec_ops_report_features: set *dev_features to NULL

gcc can get confused by this code and it thinks dev_features can be
returned uninitialized. So initialize to NULL at the beginning to shut up
the warning.

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