[ALSA] nm256 - Fix PM and irq handling

NM256 driver
- Fixed the PCM resume - restoring the rate setting
- Fixed the handling of buggy irqs
- Dynamically acquire/release irq handler to make the driver more robust
  to unknown irq storms (as OSS driver does).

Signed-off-by: Takashi Iwai <tiwai@suse.de>