ftdi: flush mpsse queue after a level change on reset pins

The function ftdi_set_signal() does not propagate the pin change
until next call to mpsse_flush(). Current code does not toggles
immediately the reset pins if polling is turned off.

Call mpsse_flush() at the end of ftdi_reset().
While there, remove the duplicated LOG message.

Change-Id: I79eacfe4fc32b5cdf2dc1b78f3660d96988466bc
Fixes: 8850eb8f2c ("swd: get rid of jtag queue to assert/deassert srst")
Reported-by: Leonard Crestez <leonard.crestez@nxp.com>
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5431
Tested-by: jenkins
Reviewed-by: Leonard Crestez <leonard.crestez@nxp.com>
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>

src/jtag/drivers/ftdi.c

@@ -550,8 +550,7 @@ static int ftdi_reset(int trst, int srst)
 			ftdi_set_signal(sig_nsrst, 'z');
 	}
 
-	LOG_DEBUG_IO("trst: %i, srst: %i", trst, srst);
-	return ERROR_OK;
+	return mpsse_flush(mpsse_ctx);
 }
 
 static void ftdi_execute_sleep(struct jtag_command *cmd)