media: Kconfig files: use the right help coding style

Checkpatch wants to use 'help' instead of '---help---':

	WARNING: prefer 'help' over '---help---' for new help texts

Let's change it globally at the media subsystem, as otherwise people
would keep using the old way.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>