TMXR: Remove output unit from delay consideration after poll has nothing queued

The goal of the logic in _tmxr_activate_delay() is to properly schedule the next
poll activity considering input and/or output data rates that may be in effect.
We need to make sure that one more poll is scheduled after output data has
been sent since simulation logic may have deferred queuing output data for
other reasons and handles its reactivation in the same context that it polls for
output.

sim_tmxr.c

@@ -2267,8 +2267,10 @@ for (i = 0; i < mp->lines; i++) {                       /* loop thru lines */
 #endif
         if ((lp->xmte == 0) && 
             ((lp->txbps == 0) ||
-             (lp->txnexttime <= sim_gtime ())))
+             (lp->txnexttime <= sim_gtime ()))) {
             lp->xmte = 1;                               /* enable line transmit */
+            lp->txnexttime = 0.0;
+            }
         }
     }                                                   /* end for */
 }