ALSA: hda - Fix Oops at reloading beep devices

The recent change for supporting dynamic beep device allocation caused
a problem resulting in Oops at reloading the driver.  Also, it ignores
the error from input device registration.

This patch fixes the wrong check in snd_hda_detach_beep_device(), and
returns an error when the input device registration fails properly.

Signed-off-by: Takashi Iwai <tiwai@suse.de>