ALSA: usb-audio: remove redundant check on err

The check on err is redundant as both the true and false paths
end up on a break statement. Remove the redundant check.

Detected by CoverityScan, CID#1268773 ("Identical code for different
branches")

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>