staging: vc04_services: use DEFINE_MUTEX() for mutex lock

mutex lock can be initialized automatically with DEFINE_MUTEX()
rather than explicitly calling mutex_init().

Signed-off-by: Zheng Yongjun <zhengyongjun3@huawei.com>
Link: https://lore.kernel.org/r/20201224132528.31558-1-zhengyongjun3@huawei.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>