[media] media: stk1160: Add Kconfig help on snd-usb-audio requirement

The Kconfig currently makes no reference to the snd-usb-audio
driver, which supports audio capture for this type of devices.
Just in case, let's make sure the requirement is mentioned
in the description.

Signed-off-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>