snd_hda_codec_read(), snd_hda_codec_write() & co take the argument
"direct" that indicates whether the given NID is a direct reference or
an indirect reference.  However, the indirect reference is practically
unimplemented and never exists.  And moreover, we don't need the
indication of indirect reference at this high level, as NID can be
represented in 16bit values at this point.

Meanwhile, there are some cases where it'd be nice to give some
operational options to these functions.  So, we can reuse this
argument as a new bit flag!  Pretty frugal, eh?

All callers so far pass zero to this argument, thus there is no
behavior change by this replacement.

The real usage of this new bit option will be added in the following
patches.
Signed-off-by: Takashi Iwai <tiwai@suse.de>