ALSA: hda - Clarify CONFIG_SND_HDA_RECONFIG usages

Since the recent rewrite of HD-audio infrastructure,
CONFIG_SND_HDA_RECONFIG has a slightly different meaning.  In the
earlier versions, it implicitly assumed only the usage via hwdep sysfs
entries.  Meanwhile, in the recent version, this option is meant to
enable the reconfig code in HD-audio driver, which may be used by the
patch loader without hwdep interface.

This patch tries to clarify the usage pattern a bit better, hopefully
avoid the further confusion.

Reported-by: Jochen Henneberg <jh@henneberg-systemdesign.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>