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.