media: mtk-vcodec: fix Kconfig help text

The Kconfig help text for VIDEO_MEDIATEK_VCODEC was referring to a
single module named mtk-vcodec, while this driver produces two modules
named mtk-vcodec-dec and mtk-vcodec-enc. Fix that.

Reported-by: Ezequiel Garcia <ezequiel@collabora.com>
Signed-off-by: Alexandre Courbot <acourbot@chromium.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>