fix buffer length var in socket_write()

Author: Bongjun Hur

adafruit_wiznet5k/adafruit_wiznet5k.py

@@ -744,11 +744,12 @@ def socket_write(self, socket_num, buffer, timeout=0):
         dst_addr = offset + (socket_num * 2048 + 0x8000)
 
         # update sn_tx_wr to the value + data size
-        ptr = (ptr + len(buffer)) & 0xFFFF
+        ptr = (ptr + ret) & 0xFFFF
         self._write_sntx_wr(socket_num, ptr)
 
         cntl_byte = 0x14 + (socket_num << 5)
-        self.write(dst_addr, cntl_byte, buffer)
+        txbuf = buffer[:ret] # <- use ret
+        self.write(dst_addr, cntl_byte, txbuf)
 
         self._write_sncr(socket_num, CMD_SOCK_SEND)
         self._read_sncr(socket_num)