V4L/DVB (3653a): Kconfig: clean up media/usb menus

- removed redundant Video For Linux API help text
- fixed dependency / selection for USB_W9968CF

Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>