tee: optee: Remove redundant custom workqueue

Global system workqueue is sufficient to suffice OP-TEE bus scanning work
needs. So drop redundant usage of the custom workqueue.

Tested-by: Jan Kiszka <jan.kiszka@siemens.com>
Tested-by: Masahisa Kojima <masahisa.kojima@linaro.org>
Signed-off-by: Sumit Garg <sumit.garg@linaro.org>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>