mwifiex: initiate card-specific work atomically

The non-atomic test + set is a little awkward here, and it technically
means we might double-schedule work unnecessarily. AFAICT, this is not
really a problem, since the extra "work" will be a no-op (the flag(s)
will be cleared by then), but it's still an anti-pattern.

Rewrite this to use the atomic test_and_set_bit() helper instead.

Signed-off-by: Brian Norris <briannorris@chromium.org>
Reviewed-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>