media: tc358743: cleanup tc358743_cec_isr

tc358743_cec_isr is misnammed, it is not the main isr.
So rename it to be consistent with its siblings,
tc358743_cec_handler.

It also does not check if its input parameter 'handled' is
is non NULL like its siblings, so add a check.

Fixes: a0ec8d1d ("media: tc358743: add CEC support")
Signed-off-by: Tom Rix <trix@redhat.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>