[media] cec: add cec_phys_addr_invalidate() helper function

Simplifies setting the physical address to CEC_PHYS_ADDR_INVALID.

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