wl12xx: cancel delayed elp work and clear flags when stopping PLT

In some cases a race condition can happen if we don't cancel any
pending ELP work before stopping PLT.  With this commit we cancel ELP
work and clear the wl->flags bitmask.  Also clean up the wl elements
after powering off.

Signed-off-by: Luciano Coelho <coelho@ti.com>