[media] pulse8-cec: set correct Signal Free Time

Don't hardcode the signal free time to 3 bit periods, instead use
the value for the signal free time as passed in by the CEC framework.

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