wl1251: don't use [delayed_]work_pending()

There's no need to test whether a (delayed) work item in pending
before queueing, flushing or cancelling it.  Most uses are unnecessary
and quite a few of them are buggy.

Remove unnecessary pending tests from wl1251.  Only compile tested.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Luciano Coelho <coelho@ti.com>
Cc: linux-wireless@vger.kernel.org