This patch extends the capability of the auto-mic feature.
Instead of limiting the automatic input-source selection only to the
mics (internal, external and dock mics), allow it for generic inputs,
e.g. switching between the rear line-in and the front mic.

The logic is to check the attribute and location of input pins, and
enable the automatic selection feature only if all such pins are in
different locations (e.g. internal, front, rear, etc) and line-in or
mic pins.  That is, if multiple input pins are assigned to a single
location, the feature isn't enabled because we don't know the
priority.

(You may wonder why this restriction doesn't exist for the headphone
 automute.  The reason is that the output case is different from the
 input: the input source is an exclusive selection while the output
 can be multiplexed.)

Note that, for avoiding regressions, the line-in auto switching
feature isn't activated as default.  It has to be set explicitly via
spec->line_in_auto_switch flag in a fixup code.
Signed-off-by: Takashi Iwai <tiwai@suse.de>