wl1251: Implement delayed entry into ELP mode

Implement (slightly) delayed entry into ELP. This will cure several
problems:
  - It works around a firmware race condition if ELP is entered too fast
    after commands (resulting in ELP timeout -traces)
  - It will reduce the number of sleep-wake cycles between already
    scheduled events such as interrupts and tx, hence improving
    performance (less delay in switching between RX and TX)

Signed-off-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Reviewed-by: Vidhya Govindan <vidhya.govindan@nokia.com>
Signed-off-by: Kalle Valo <kalle.valo@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/wl12xx/wl1251_ps.h

@@ -31,6 +31,7 @@
 int wl1251_ps_set_mode(struct wl1251 *wl, enum wl1251_cmd_ps_mode mode);
 void wl1251_ps_elp_sleep(struct wl1251 *wl);
 int wl1251_ps_elp_wakeup(struct wl1251 *wl);
+void wl1251_elp_work(struct work_struct *work);
 
 
 #endif /* __WL1251_PS_H__ */