V4L/DVB (9979): em28xx: move usb probe code to a proper place

em28xx-video were holding several code that are not specific to V4L2
interface.

This patch moves the core code for em28xx-core, and usb probing code
into em28xx-cards.

This opens the possibility of breaking em28xx into a core module and a
V4L2 module, loaded only on devices that have analog interfaces.

Some cleanup may be done at em28xx-cards to optimize the config code.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>