ALSA: hda - Support indirect execution of verbs

Add an overriding exec_verb op to struct hdac_device so that the call
via snd_hdac_exec_verb() can switch to a different route depending on
the setup.  The codec driver sets this field so that it can handle the
errors or applying quirks appropriately.  Furthermore, this mechanism
will be used for smooth transition for the regmap support in later
patches.

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