[media] cec: fix Kconfig help text

MEDIA_CEC is no longer a tristate option, so the user can't actually
choose M. Whether the code is built-in or built as a module is
decided somewhere else.

Fixes: 5bb2399a ("[media] cec: fix Kconfig dependency problems")

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>