staging: rtl8188eu: use standard kthread handling

staging: rtl8188eu: use standard kthread handling

The driver defines its own completion to wait until the command thread
is stopped. Use kthread_stop instead and check kthread_should_stop in
the thread's main loop. (For now, we keep the driver's "emergency exit"
via bDriverStopped/bSurpriseRemoved.)

To check if the command thread is running, the driver stores the thread's
task_struct and a boolean status variable. Remove this status and check the
task_struct directly.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20210605165858.3175-8-martin@kaiser.cx
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>