ion: Use correct heap when freeing secure system heap buffers

Switch the heap for buffers from the secure system heap in
the free path, so that they can be properly released into
the secure pools.

Change-Id: I9873d73efa6f56a2c2ceb8d29e4b594ff30b9bca
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>