[ALSA] cmipci: add msbits constraint for 24-bit format

Add a msbits constraint to the SPDIF output device to indicate that
S32_LE samples use only 24 bits for data.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>