The operating system is Debian. This is a dirty solution but it does work anyway.
root@beaglebone:~# cat /etc/pm/power.d/wlan0_pm_off
#!/bin/bash
[ -x /sbin/iwconfig ] || exit 0
[ -n "`/sbin/iwconfig 2>/dev/null | grep wlan0`" ] || exit 0
/sbin/iwconfig wlan0 power off
root@beaglebone:~# crontab -l | grep -v "^#"
*/1 * * * * /etc/pm/power.d/wlan0_pm_off
root@beaglebone:~# iwconfig wlan0
wlan0 IEEE 802.11abgn ESSID:"*****"
...
Power Management:off
...
