Revert read_string() to partial results are a success status

The read_string() function was changed to return a -RIG_ETIMEOUT error
when timing out after having read  some characters. This caused a back
end to fail because it was  using a read_string() with an invalid stop
character  and relying  on the  timed out  read_string() to  fetch the
data. This  patch reverts to  the prior  behavior of returning  a null
terminated buffer and  read bytes count if at least  one byte has been
read.

src/iofunc.c

@@ -505,6 +505,7 @@ int HAMLIB_API read_string(hamlib_port_t *p, char *rxbuffer, size_t rxmax, const
 
 	retval = port_select(p, p->fd+1, &rfds, NULL, &efds, &tv);
 	if (retval == 0) {
+    if (0 == total_count) {
       /* Record timeout time and caculate elapsed time */
       gettimeofday(&end_time, NULL);
       timersub(&end_time, &start_time, &elapsed_time);
@@ -515,6 +516,8 @@ int HAMLIB_API read_string(hamlib_port_t *p, char *rxbuffer, size_t rxmax, const
 
       return -RIG_ETIMEOUT;
     }
+    break;                      /* return what we have read */
+  }
 
 	if (retval < 0) {
 		dump_hex((unsigned char *) rxbuffer, total_count);