USB: ftdi_sio: fix DTR/RTS line modes

Call set_mctrl() and clear_mctrl() according to the flow control mode
selected. This makes serial communication for FT232 connected devices
work when CRTSCTS is not set.

This fixes a regression introduced by 4175f3e3 ("tty_port: If we are
opened non blocking we still need to raise the carrier"). This patch
calls the low-level driver's dtr_rts() function which consequently sets
TIOCM_DTR | TIOCM_RTS. A later call to set_termios() without CRTSCTS in
cflags, however, does not reset these bits, and so data is not actually
sent out on the serial wire.

Signed-off-by: Daniel Mack <daniel@caiaq.de>
Cc: Johan Hovold <jhovold@gmail.com>
Cc: Alan Cox <alan@linux.intel.com>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>