wl12xx: sleep instead of wakeup after tx work

commit d05c806 ("wl12xx: rearrange some ELP wake_up/sleep calls")
introduced a bug in which wl1271_ps_elp_wakeup() was called instead
of wl1271_ps_elp_sleep() after completing the tx work.

Reported-by: Arik Nemtsov <arik@wizery.com>
Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>