eac3dec: fix scaling

This is the remaining error, the output on the SPX samples,
respectively csi_miami_stereo_128_spx.eac3 and
csi_miami_5.1_256_spx.eac3, goes from:
stddev:    8.71 PSNR: 77.52 MAXDIFF:  235
stddev:24270.51 PSNR: 22.17 MAXDIFF:47166
to:
stddev:    0.12 PSNR:114.12 MAXDIFF:    1
stddev:    0.12 PSNR:114.73 MAXDIFF:    1

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>