ion: Fix typo in ion system secure heap

Correct the function pointer used to unmap.
This is not expected to have any functional impact, as both the new and
old function pointers were nops.

CRs-Fixed: 997730
Change-Id: I2f78d5c798d0d2575d52b341aaf29b35294c93f1
Reported-by: Rohit Kulkarni <rkulkarn@codeaurora.org>
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>