swr: add streamout buffer offset into pBuffer pointer

swr: add streamout buffer offset into pBuffer pointer

The buffer_size does not take the offset into account. Just add the
offset into the pointer which lines up the structures much better.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Tim Rowley <timothy.o.rowley@intel.com>