[ALSA] maestro3.c: fix BUG, optimization

- fix brown-paper-bag locking bug (lock() / return / unlock())
- improve central function snd_m3_update_ptr()
  (avoid expensive integer divisions)
- add cpu_relax() to busy-wait I/O loop as recommended
  (does this require special macro support in ALSA for older kernels??)
- constify several structs
- spelling updates

Signed-off-by: Andreas Mohr <andi@lisas.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>