Set wlvif->ps_compl to NULL in before return

wl1271_configure_suspend_sta leaves a stale stack declared
completion in wlvif->ps_compl. Set it to NULL before returning.

Signed-off-by: Pontus Fuchs <pontus.fuchs@gmail.com>
[small fix to use wlvif->ps_compl instead of wl->ps_compl]
Signed-off-by: Luciano Coelho <coelho@ti.com>