ALSA: usb-audio: Fix build with CONFIG_INPUT=n

ALSA: usb-audio: Fix build with CONFIG_INPUT=n

The recent addition of DualSense mixer quirk relies on the input
device handle, and the build can fail if CONFIG_INPUT isn't set.
Put (rather ugly) workarounds to wrap with IS_REACHABLE() for avoiding
the build error.

Fixes: 79d561c4ec04 ("ALSA: usb-audio: Add mixer quirk for Sony DualSense PS5")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202506130733.gnPKw2l3-lkp@intel.com/
Reviewed-by: Cristian Ciocaltea <cristian.ciocaltea@collabora.com>
Link: https://patch.msgid.link/20250613081543.7404-1-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>