ALSA: hda - Select INPUT for Realtek HD-audio codec

The commit commit [33f4acd3: ALSA: hda - Enable mic mute hotkey
and LEDs for an HP machine] introduced a quirk for a HP machine
involving with the input event handling.  Although the relevant code
is protected via IS_ENABLED(CONFIG_INPUT), this doesn't suffice when
the audio driver is built in while the input is module.

As an easy workaround, this patch forcibly selects CONFIG_INPUT in
Kconfig.  This shouldn't be a practical problem since CONFIG_INPUT is
almost mandatory for all systems.  Also, this allows to remove the
ugly ifdefs in the code.

Fixes: 33f4acd3 ('ALSA: hda - Enable mic mute hotkey and LEDs for an HP machine')
Acked-by: David Henningsson <david.henningsson@canonical.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>