Use a tty_port common instead. This saves lots of .text and makes the
code a lot more readable.

This involves separation of a dtr_rts handling, next patches will use
that to not duplicate the code all over the place.
Signed-off-by: NJiri Slaby <jirislaby@gmail.com>
Signed-off-by: NAlan Cox <alan@linux.intel.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>