V4L/DVB (7602): em28xx: generalise URB setup code

Move the URB setup and management code to em28xx-core.c and generalise
it slighlty so that the DVB code can use it.

Signed-off-by: Aidan Thornton <makosoft@googlemail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>