optee: fix kfree NULL pointer

This patch fixes the following Coccinelle error:
drivers/tee/optee/ffa_abi.c: 877: ERROR  optee is NULL but dereferenced.

If memory allocation fails, optee is null pointer. the code will goto err
and release optee.

Fixes: 4615e5a3 ("optee: add FF-A support")
Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: Lv Ruyi <lv.ruyi@zte.com.cn>
Reviewed-by: Sumit Garg <sumit.garg@linaro.org>
[jw: removed the redundant braces]
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>