ALSA: hda - Initialize special cases for input src in init phase

Currently some special handling for the unusual case like dual-ADCs
or a single-input-src is done in the tree-parse time in
set_capture_mixer().  But this setup could be overwritten by static
init verbs.

This patch moves the initialization into the init phase so that
such input-src setup won't be lost.

Cc: <stable@kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>