The lowlevel register read/write don't have to be careful about the
connection state.  It should be checked in the caller side instead.
By dropping the check, we can simplify the code, and readability.

This patch also refacors the functions slightly: namely,
- drop the useless always-zero return values
- fold the inline functions to the main accessor functions themselves
- move the DP audio hack for AUD_CONFIG to the caller side
- simplify snd_intelhad_eanble_audio() and drop the unused
  had_read_modify()
Signed-off-by: Takashi Iwai <tiwai@suse.de>