[media] em28xx-audio: split URB initialization code

The URB calculus code may eventually be moved to some other
place, like at pcm open, if it ends by needing more setups, like
working with different bit rates, or different audio latency.

So, move it into a separate routine. That also makes the code
more readable.

No functional changes.

Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>