USB: serial: drop unused ASYNC flags

Do not report ASYNC_SKIP_TEST or ASYNC_AUTO_IRQ as being set in
TIOCGSERIAL handlers as these flags are not supported and do not really
make any sense for USB serial devices in the first place.

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>