ALSA: hda - clean up code to reset hda link

This patch is a cleanup to the previous patch "reset hda link during system/
runtime suspend".

In this patch
- azx_enter_link_reset() and azx_exit_link_reset() are defined for entering and
  exiting the link reset respectively. azx_link_reset() is no longer used and
  replaced by azx_enter_link_reset().
- azx_reset() reuses the above two new functions for a link reset cycle

Signed-off-by: Mengdong Lin <mengdong.lin@intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>