[ALSA] au1x00 - Code clean up

Modules: MIPS AU1x00 driver

Clean up snd-au1x00 driver code:

- Remove global variables
- Remove old compatibility codes
- Fix DMA-link allocation/release functions in hw_params and hw_free
  callbacks (they may be called multiple times)
- Fix spinlocks
Signed-off-by: Takashi Iwai <tiwai@suse.de>