ALSA: sound/drivers/vx: fix sparse warning: different signedness

Fix this sparse warning:
  sound/drivers/vx/vx_uer.c:301:42: warning: incorrect type in argument 2 (different signedness)

Signed-off-by: Hannes Eder <hannes@hanneseder.net>
Signed-off-by: Takashi Iwai <tiwai@suse.de>