media: em28xx-cards: rework the em28xx probing code

There is a complex loop there with identifies the em28xx
endpoints. It has lots of identations inside, and big names,
making harder to understand.

Simplify it by moving the main logic into a static function.

While here, rename "interface" var to "intf".

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