ALSA: snd-usb: move code from urb.c to endpoint.c

No code altered at this point, simply preparing for upcoming
refactorizations.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Acked-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>