wl1271: Implement delayed entry into ELP

Implement delayed entry into ELP. This will promote the following:
 - Less redundant sleep/wake cycles (better perf)
 - Avoids known firmware issues with going to ELP too fast after an
   operation

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