[ALSA] Reduce interrupt latency in sound/pci/via82xx.c

[ALSA] Reduce interrupt latency in sound/pci/via82xx.c

Modules: VIA82xx driver

The change only affects the via823x kind of chips.
Here the  via8233_pcm_pointer_hw() function
(named snd_via8233_pcm_pointer() before)
needed to loop until a non zero position is red from the chip.

Measurements have shown that more than 200 loops are typically needed on
an Athlon64.
As io-reads cost many cycles, those loops sum up huge.
via8233_pcm_pointer_hw() runs either in interrupt or with interrupts
disabled. So it introduces significant interrupt latency.

The patch introduces a calculated position value hwptr_done,
that is updated by the interrupt routine when a period is completed.
It is only used, if the 823x chip returns a zero position, which can't
be interpreted reliably.

Further optimisation is applied on the 8233 chip's interrupt routine:
Only the SGD_SHADOW is read, as it contains all infos needed.
We ommit ~5 more register reads that way.

Signed-off-by: Karsten Wiese <annabellesgarden@yahoo.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>