qseecom: simplify unload app operation

simplify qseecom_unload_app operation to first decrease app loading
ref_cnt, then unload app when ref_cnt is 0.

Change-Id: I687c3f01fea0644b0b33d1b23b8438f38e58bffb
Signed-off-by: Zhen Kong <zkong@codeaurora.org>