xerox_mfp: Mark SCX-4x16 as unsupported

As reported by Andrew Sotnikov in issue #95 Samsung SCX-4216F is not
really supported. Change SCX-4x16 status from untested to unsupported.
Closes #95.

doc/descriptions/xerox_mfp.desc

@@ -87,7 +87,7 @@
 :model "SCX-4x16"
 :interface "USB"
 :usbid "0x04e8" "0x3409"
-:status :untested
+:status :unsupported
 
 :model "SCX-6x20"
 :interface "USB"