msm: Return -ENOSYS when secure buffer apis are not implemented

The return value -EINVAL is returned in the case of invalid
arguments, and is not the correct value when the function is not
implemented.

Return -ENOSYS instead.

Change-Id: I196537f121d5a290fec74e2b7bcb1cfd490468c7
Signed-off-by: Neeti Desai <neetid@codeaurora.org>