firmware: arm_scmi: optee: Fix missing mutex_init()

The driver allocates the mutex but not initialize it.
Use mutex_init() on it to initialize it correctly.

Link: https://lore.kernel.org/r/20211123083620.2366860-1-weiyongjun1@huawei.com


Fixes: 5f90f189 ("firmware: arm_scmi: Add optee transport")
Reported-by: Hulk Robot <hulkci@huawei.com>
Reviewed-by: Etienne Carriere <etienne.carriere@linaro.org>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>