media: cec-pin: fix off-by-one SFT check

The CEC pin framework has to wait for the CEC bus to be idle for the
requested Signal Free Time before it can start a transmit.

However, the check for that was off by one, so transmits would start one
bit period (2.4ms) too late.

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