Avoid wrong calculation of payload buffer size for media

While calculating the out MTU size for a A2DP packet, the AVDT header
size is subtracted twice results in wrong calculation. To avoid this the
size is subtracted only once.

Crs-Fixed: 675980
Change-Id: Idf8dd4025698b290837cc0031a18b654717ff6ed
Merge branch 'cm' into HEAD