qseecom: fixed a bug in __qseecom_set_sb_memory

__qseecom_set_sb_memory should only unmap
address in error case, and return directly
in case of success to avoid unnecessary
unmap.

Change-Id: I96c81ccfb8fb76613b911ac457b588050c43f7e9
Signed-off-by: Zhen Kong <zkong@codeaurora.org>