msm: ion: Do not call unassign on buffer if assign fails

During buffer allocation if the assign call fails, we call
ion_system_secure_heap_free, which in turn does an unassign
operation before freeing the buffer. We don't want to call
unassign if the assign failed.

Fix this by calling the underlying free function directly
during error handling.

Change-Id: I2deec6fd147c4200e16f3779e37330dca1d7d173
Signed-off-by: Neeti Desai <neetid@codeaurora.org>