tee: optee: smc: remove unnecessary NULL check before release_firmware()

release_firmware() checks for NULL pointers internally.
Remove unneeded NULL check for fmw here.

Signed-off-by: Chen Ni <nichen@iscas.ac.cn>
Reviewed-by: Sumit Garg <sumit.garg@oss.qualcomm.com>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>