You need to get the driver from Ralink's web site:

http://www.ralinktech.com.tw/drivers...1.0.3.6.tar.gz

You will have to edit rtmp_def.h and add the F5D7050 device to the #define RT73_USB_DEVICES section:

{USB_DEVICE(0x050d,0x705a)}, /* Ralink */ \

Then compile and install the driver which will be called rausb0.