media: cec.h: add cec_msg_recv_is_rx/tx_result helpers

These two helper functions return true if the received message
contains the result of a previous non-blocking transmit. Either
the tx_status result (cec_msg_recv_is_tx_result) of the transmit,
or the rx_status result (cec_msg_recv_is_rx_result) of the reply
to the original transmit.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>