avcodec/(e)ac3: Fix target_level for EAC3.

Currently when using target_level with EAC3 it produces silence. This small patch fixes target_level for decoding EAC3.

Example:
ffmpeg -y -i /tmp/test.wav -acodec eac3 -dialnorm -14 -ac 6 -b:a 384000 /tmp/test.m2ts
ffmpeg -y -target_level -24 -i /tmp/test.m2ts -acodec pcm_s16le -f matroska /tmp/out.mkv
ffplay /tmp/out.mkv

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>