soc: qcom: mem-buf: Do not free memory if hyp_assign() fails

The state of memory--with respect to access control--is unknown
when a hyp_assign() call fails because the underlying SCM call
fails. In these cases, it is not safe to access the memory, so
we should not free it back to the system.

Change-Id: I992225cdd5f26fbba60d2966d051d026326cd593
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>