refine abort sequence

homing the head with "G28" does not work on my printer for some reason. "G28 X" and "G28 Y" however do.
this fix should not break behaviour on other printers as "G28" is just a shortcut.

plugins/USBPrinting/USBPrinterOutputDevice.py

@@ -119,7 +119,8 @@ class USBPrinterOutputDevice(PrinterOutputDevice):
         self._sendCommand("G0 Y%s F%s" % (z, speed))
 
     def _homeHead(self):
-        self._sendCommand("G28")
+        self._sendCommand("G28 X")
+        self._sendCommand("G28 Y")
 
     def _homeBed(self):
         self._sendCommand("G28 Z")