ALSA: usb-audio - Fix types taken in min()

Fix the compile warning due to different integer types used in min():
  sound/usb/usbaudio.c: In function 'init_substream_urbs':
  sound/usb/usbaudio.c:1087: warning: comparison of distinct pointer types lacks a cast

Signed-off-by: Takashi Iwai <tiwai@suse.de>