ARM: OMAP: serial: remove last vestige of DTR_gpio support.

These fields were added by:
commit 9574f36f
    OMAP/serial: Add support for driving a GPIO as DTR.

but not removed by

commit 985bfd54
    tty: serial: omap: remove some dead code

which reverted most of that commit.
Time to revert the rest.

Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Tony Lindgren <tony@atomide.com>