media: cec: core: don't set last_initiator if tx in progress

When a message was received the last_initiator is set to 0xff.
This will force the signal free time for the next transmit
to that for a new initiator. However, if a new transmit is
already in progress, then don't set last_initiator, since
that's the initiator of the current transmit. Overwriting
this would cause the signal free time of a following transmit
to be that of the new initiator instead of a next transmit.

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