rt21: Fix DTCS decoding when stored in hex

Apparently this radio has a bit for whether or not the DTCS code is
stored in hex or octal. CHIRP was only supporting octal format,
but radios with codes stored in hex were found in the wild and we,
of course, fail to decode those.

Fixes #11749

chirp/drivers/retevis_rt21.py

@@ -839,8 +839,16 @@ def get_raw_memory(self, number):
         return repr(self._memobj.memory[number - 1])
 
     def _get_tone(self, _mem, mem):
+        # 0xA662 = 662R = 1010 0...
+        # 0xA9DC = 734R = 1010 1...
+        # 0x29D3 = 723N - 0010 1...
+
         def _get_dcs(val):
-            code = int("%03o" % (val & 0x07FF))
+            if val & 0x0800:
+                code = int("%03o" % (val & 0x07FF))
+            else:
+                # 0x0800 means code is stored in hex instead of ocal (?!)
+                code = int("%03x" % (val & 0x07FF))
             pol = (val & 0x8000) and "R" or "N"
             return code, pol