optee: Fix NULL but dereferenced coccicheck error

Eliminate the following coccicheck warning:
./drivers/tee/optee/smc_abi.c:1508:12-15: ERROR: optee is NULL but
dereferenced.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Fixes: 6749e69c ("optee: add asynchronous notifications")
Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>