isdn: i4l: Remove ASYNC_CTS_FLOW

ISDN4Linux always enables CTS flow control and does not use the
tty_port_cts_enabled() helper function; remove ASYNC_CTS_FLOW
state enable/disable.

cc: Karsten Keil <isdn@linux-pingi.de>
cc: <netdev@vger.kernel.org>
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>