[media] cec: fix off-by-one memset

The unused bytes of the features array should be zeroed, but the start index was one
byte too early. This caused the device features byte to be overwritten by 0.

The compliance test for the CEC_S_LOG_ADDRS ioctl didn't catch this because it tested
byte continuation with the second device features byte being 0 :-(

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