Avoid redefining MSTP timeouts (#230)

* Avoid redefining MSTP timeouts

* Remove duplicate MS/TP time definitions

Ports of MS/TP datalink included #defines of several time values, which
were already defined in src/bacnet/datalink/mstpdef.h.  All those ports
already include that header.

ports/atmega168/dlmstp.c

@@ -139,13 +139,17 @@ static uint8_t TransmitPacketDest;
 /* that a node must wait for a station to begin replying to a */
 /* confirmed request: 255 milliseconds. (Implementations may use */
 /* larger values for this timeout, not to exceed 300 milliseconds.) */
+#ifndef Treply_timeout
 #define Treply_timeout 295
+#endif
 
 /* The time without a DataAvailable or ReceiveError event that a node must */
 /* wait for a remote node to begin using a token or replying to a Poll For */
 /* Master frame: 20 milliseconds. (Implementations may use larger values for */
 /* this timeout, not to exceed 35 milliseconds.) */
+#ifndef Tusage_timeout
 #define Tusage_timeout 30
+#endif
 
 /* The minimum number of DataAvailable or ReceiveError events that must be */
 /* seen by a receiving node in order to declare the line "active": 4. */
@@ -157,12 +161,16 @@ static uint8_t TransmitPacketDest;
 /* not to exceed 100 milliseconds.) */
 /* At 9600 baud, 60 bit times would be about 6.25 milliseconds */
 /* const uint16_t Tframe_abort = 1 + ((1000 * 60) / 9600); */
+#ifndef Tframe_abort
 #define Tframe_abort 30
+#endif
 
 /* The maximum time a node may wait after reception of a frame that expects */
 /* a reply before sending the first octet of a reply or Reply Postponed */
 /* frame: 250 milliseconds. */
+#ifndef Treply_delay
 #define Treply_delay 250
+#endif
 
 /* we need to be able to increment without rolling over */
 #define INCREMENT_AND_LIMIT_UINT8(x) \