firmware: use the calculated size when reading the next chunk of an ota file

This commit fixes the size of the next ota file chunk to use the calculated
one instead of the default one.

This was causing a 'Data payload too large' transmit error (0x74) when
transmitting the first ota packet in DigiMesh (fw versions 3004 and 3003).

Signed-off-by: Tatiana Leon <tatiana.leon@digi.com>

Author: tatianaleon

digi/xbee/firmware.py

@@ -289,7 +289,7 @@ def get_next_data_chunk(self):
                 self._file = open(self._file_path, "rb")
                 self._file.read(self._discard_size)
 
-            return self._file.read(_OTA_DEFAULT_BLOCK_SIZE)
+            return self._file.read(self._chunk_size)
         except IOError as e:
             self.close_file()
             raise _ParsingOTAException(str(e))