[ALSA] ali5451 - Code clean up, irq handler fix

- Clean up ali5451.c, following the standard coding style, unneeded codes
  reduced, and removal of redundant variable initializations.
  Hungarian notation isn't fixed yet ;)
- Fix irq handler to return IRQ_NONE properly for shared irqs.
  Also check the hardware availability in irq handler to avoid possible
  initialization races at loading the driver.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>