media: cec-adap: fix regression in ping sanity check

Commit b6c96e15 inadvertently also dropped the 'msg->len > 1' test
from the preceding sanity check. This caused compliance test failures.

Fixes: b6c96e15 ("media: cec: allow any initiator for Ping and Image/Text View On")

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