remoteproc: imx_dsp_rproc: Add mutex protection for workqueue

In the Linux kernel, the following vulnerability has been resolved:

remoteproc: imx_dsp_rproc: Add mutex protection for workqueue

The workqueue may execute late even after remoteproc is stopped or
stopping, some resources (rpmsg device and endpoint) have been
released in rproc_stop_subdevices(), then rproc_vq_interrupt()
accessing these resources will cause kennel dump.

Call trace:
virtqueue_add_split+0x1ac/0x560
virtqueue_add_inbuf+0x4c/0x60
rpmsg_recv_done+0x15c/0x294
vring_interrupt+0x6c/0xa4
rproc_vq_interrupt+0x30/0x50
imx_dsp_rproc_vq_work+0x24/0x40 [imx_dsp_rproc]
process_one_work+0x1d0/0x354
worker_thread+0x13c/0x470
kthread+0x154/0x160
ret_from_fork+0x10/0x20

Add mutex protection in imx_dsp_rproc_vq_work(), if the state is
not running, then just skip calling rproc_vq_interrupt().

Also the flush workqueue operation can't be added in rproc stop
for the same reason. The call sequence is

rproc_shutdown
-> rproc_stop
->rproc_stop_subdevices
->rproc->ops->stop()
->imx_dsp_rproc_stop
->flush_work
-> rproc_vq_interrupt

The resource needed by rproc_vq_interrupt has been released in
rproc_stop_subdevices, so flush_work is not safe to be called in
imx_dsp_rproc_stop.