wl1271: set null data template when BSSID is known

The call to wl1271_cmd_build_null_data() was missing when we got associated,
this was causing PS to fail.  This patch adds the call and now PS seems to
work.

Signed-off-by: Luciano Coelho <luciano.coelho@nokia.com>
Reviewed-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>