wfx: avoid flush_workqueue(system_highpri_wq) usage

Flushing system-wide workqueues is dangerous and will be forbidden.
Replace system_highpri_wq with per "struct wfx_dev" bh_wq.

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Acked-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/f15574a6-aba4-72bc-73af-26fdcdf9fb63@I-love.SAKURA.ne.jp