ALSA: aloop: Support S24 sample formats

Currently snd_aloop supports only S16 and S32 audio sample formats. With
this patch the S24 formats are also supported.

Signed-off-by: Timo Wischer <twischer@de.adit-jv.com>
Reviewed-by: Jaroslav Kysela <perex@perex.cz>
Signed-off-by: Takashi Iwai <tiwai@suse.de>