ALSA: hda - Use IS_REACHABLE() for dependency on input

The commit ffcd28d8 ("ALSA: hda - Select INPUT for Realtek
HD-audio codec") introduced the reverse-selection of CONFIG_INPUT for
Realtek codec in order to avoid the mess with dependency between
built-in and modules.  Later on, we obtained IS_REACHABLE() macro
exactly for this kind of problems, and now we can remove th INPUT
selection in Kconfig and put IS_REACHABLE(INPUT) to the appropriate
places in the code, so that the driver doesn't need to select other
subsystem forcibly.

Fixes: ffcd28d8 ("ALSA: hda - Select INPUT for Realtek HD-audio codec")
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org> # and build-tested
Signed-off-by: Takashi Iwai <tiwai@suse.de>