ALSA: xen-front: remove redundant error check on ret

ALSA: xen-front: remove redundant error check on ret

The error for a -ve value in ret is redundant as all previous
assignments to ret have an associated -ve check and hence it
is impossible for ret to be less that zero at the point of the
check.  Remove this redundant error check.

Detected by CoveritScan, CID#1469407 ("Logically Dead code")

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>