ALSA: x86: Yet more tidy-up and clean-ups

- Add a few more comments to functions.

- Move the initialization of some PCM state variables to open and
  prepare callbacks, where these are clearer places.

- Remove superfluous NULL checks.

- Get rid of the bogus drv_status change to CONNECTED at close;
  this doesn't make any sense.

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