This device does not support master mode and also does not support WPA access point mode. They support monitor mode (only through reports on the internet have I learned this). This makes them useless for hostapd.
I did little testing in AP mode via WEP. I did not get decent speeds (nothing over 1MB a second) usually around 400k and they would disconnect all the time in AP mode.
lsusb | grep -i wireless Bus 001 Device 019: ID 148f:3070 Ralink Technology, Corp. RT2870/RT3070 Wireless Adapter
They work in master mode via hostapd.