Create file /etc/modprobe.d/8192cu.conf
$ sudo nano /etc/modprobe.d/8192cu.conf
add this line
options 8192cu rtw_power_mgnt=0 rtw_enusbss=0
Restart raspberry pi
Reference :
https://github.com/xbianonpi/xbian/issues/217
https://github.com/pvaret/rtl8192cu-fixes/blob/master/8192cu-disable-power-management.conf
