... then don't start usbnetwork automatically
Update: sorry, that was too fast. It seems like it's indeed impossible to have both the USB drive attached and WLAN working (WLAN seems to be killed on USB drive connect). However, you can still "disable" usbnetwork (no need to auto-enable it if you don't want it enabled, duh...). SSH access via Wifi should still work, as long as you don't plug the device to the computer. If you plug it in, Wifi is killed, but it will show up as a USB drive.