wl1251: decrease bt coex before beacon time

wl1251: decrease bt coex before beacon time

With the current setup bluetooth coexistance consumes too much power
because firmware wakes up for beacon really early. As a workaround reduce
the wakeup time to 500 us to save power. This patch will be reverted when
a proper solution for bluetooth coexistance high power consumption is
implemented.

This decreases wlan idle power consumption from ~22 mA to ~14 mA.

Signed-off-by: Kalle Valo <kalle.valo@nokia.com>
Reviewed-by: Vidhya Govindan <vidhya.govindan@nokia.com>
Signed-off-by: Luciano Coelho <luciano.coelho@nokia.com>
Signed-off-by: Yuri Ershov <ext-yuri.ershov@nokia.com>