stagefright aacenc: Fix an inline asm constraint

Output-only parameters should use =r or =&r constrains, never +r.

This avoids a warning about the variable 'result' being used
uninitialized.

This avoids loading the uninitialized value into the register,
before it is overwritten by the multiplication.

Change-Id: I0ef6179e133c35d290feb8e12bea180ecae11a05