usb/serial: Remove unnecessary check for console

The tty port ops shutdown() routine is not called for console ports;
remove extra check.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Acked-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>