wl1251: cancel elp_work early

wl1251: cancel elp_work early

While working on PS I've noticed elp_work is kicking rather often, and
sometimes the chip is put to sleep before 5ms delay expires. This
seems to happen because by the time wl1251_ps_elp_wakeup is called
elp_work might still be pending, and if the processing takes longer
(for example interrupts may take some time to process), elp_work might
get started and hit the mutex. After main thread finishes work it
calls wl1251_ps_elp_sleep, which can no longer cancel old work (as
it's already started) and schedules yet another work needlessly. The
elp_work then gets the mutex and puts the chip to sleep too early.

Fix this by cancelling work in wl1251_ps_elp_wakeup instead.

Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>