media: pulse8-cec: fix lost cec_transmit_attempt_done() call

The periodic PING command could interfere with the result of
a CEC transmit, causing a lost cec_transmit_attempt_done()
call.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Cc: <stable@vger.kernel.org>      # for v4.10 and up
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>