ALSA: ali5451: remove irq enabling in pointer callback

snd_ali_pointer function is called with local
interrupts disabled. However it seems very strange to
reenable them in such way.

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Denis Efremov <yefremov.denis@gmail.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>