imc: Use correct position for flcoeffs2 calculation

flcoeffs2[pos] should be the log2 of flcoeffs1[pos].
flcoeffs1[0] can be 0 here, thus flcoeffs2[pos] gets set to -inf,
causing problems further down.

This seems to have been copied from imc_decode_level_coefficients in
commit 4eb4bb3a without updating the position.

Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>