wwan_hwsim: Avoid flush_scheduled_work() usage

Flushing system-wide workqueues is dangerous and will be forbidden.
Replace system_wq with local wwan_wq.

While we are at it, make err_clean_devs: label of wwan_hwsim_init()
behave like wwan_hwsim_exit(), for it is theoretically possible to call
wwan_hwsim_debugfs_devcreate_write()/wwan_hwsim_debugfs_devdestroy_write()
by the moment wwan_hwsim_init_devs() returns.

Link: https://lkml.kernel.org/r/49925af7-78a8-a3dd-bce6-cfc02e1a9236@I-love.SAKURA.ne.jp


Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Reviewed-by: Sergey Ryazanov <ryazanov.s.a@gmail.com>
Reviewed-by: Loic Poulain <loic.poulain@linaro.org>
Link: https://lore.kernel.org/r/7390d51f-60e2-3cee-5277-b819a55ceabe@I-love.SAKURA.ne.jp


Signed-off-by: Jakub Kicinski <kuba@kernel.org>