ft857: Fix incorrect DTCS handling

This driver has been broken forever in how it handles DTCS tone mode.
It was always using the rx_dtcs field, which should only be honored
in a ->DTCS cross mode. The new GUI, being in hide-unused-fields mode
always made this clear, as it appeared to be snapping back to 023
always when the user selected a different value.

Note that the test is *also* broken, which I will fix at the end of
this series.

Related to #11506

chirp/drivers/ft857.py

@@ -514,7 +514,7 @@ def _set_tmode(self, mem, _mem):
             # this code has already been tested
             _mem.dcs = _mem.rxdcs = chirp_common.DTCS_CODES.index(mem.dtcs)
         elif mem.tmode == "DTCS":
-            _mem.dcs = _mem.rxdcs = chirp_common.DTCS_CODES.index(mem.rx_dtcs)
+            _mem.dcs = _mem.rxdcs = chirp_common.DTCS_CODES.index(mem.dtcs)
         elif mem.tmode == "Cross":
             _mem.tone = chirp_common.TONES.index(mem.rtone)
             _mem.rxtone = chirp_common.TONES.index(mem.ctone)