optee: simplify optee_release()

Simplifies optee_release() with a new helper function,
optee_close_session_helper() which has been factored out from
optee_close_session().

A separate optee_release_supp() is added for the supplicant device.

Reviewed-by: Sumit Garg <sumit.garg@linaro.org>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>