alsdec: limit avctx->bits_per_raw_sample to 32

avctx->bits_per_raw_sample is used in get_sbits_long, which only
supports up to 32 bits.

Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
