USB: serial: console: add support for flow control

Add support for enabling hardware flow control using the 'r' command
line option.

This also avoids a W=1 (-Wunused-but-set-variable) warning.

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