ALSA: ali5451: Fix out-of-bound position reporting

The pointer callbacks of ali5451 driver may return the value at the
boundary occasionally, and it results in the kernel warning like
  snd_ali5451 0000:00:06.0: BUG: , pos = 16384, buffer size = 16384, period size = 1024

It seems that folding the position offset is enough for fixing the
warning and no ill-effect has been seen by that.

Reported-by: Enrico Mioso <mrkiko.rs@gmail.com>
Tested-by: Enrico Mioso <mrkiko.rs@gmail.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>