serial: simplify tsr_retry reset

Move common code outside the if, and reset tsr_retry even in loopback mode.
Right now it cannot become non-zero, but it will be possible as soon as
we start respecting the baud rate.

Tested-by: Bret Ketchum <bcketchum@gmail.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

hw/char/serial.c

@@ -258,10 +258,8 @@ static gboolean serial_xmit(GIOChannel *chan, GIOCondition cond, void *opaque)
                 s->tsr_retry++;
                 return FALSE;
             }
-            s->tsr_retry = 0;
-        } else {
-            s->tsr_retry = 0;
         }
+        s->tsr_retry = 0;
 
         /* Transmit another byte if it is already available. It is only
            possible when FIFO is enabled and not empty. */