serial: fix rs485 for atmel_serial on avr32

This patch fixes a build failure [1-4] in the atmel_serial code introduced by
patch the patch ARM: 6092/1: atmel_serial: support for RS485
communications (e8faff73)

The build failure was caused by missing struct field and missing defines
for the avr32 board - the patch fixes this.

[1] http://kisskb.ellerman.id.au/kisskb/buildresult/2575242/ - first failure in linux-next, may 11th
[2] http://kisskb.ellerman.id.au/kisskb/buildresult/2816418/ - still exists as of today
[3] http://kisskb.ellerman.id.au/kisskb/buildresult/2617511/ - first failure in Linus' tree - May 20th - did really no one notice this?!
[4] http://kisskb.ellerman.id.au/kisskb/buildresult/2813956/

 - still exists in Linus' tree as of today

Signed-off-by: Peter Huewe <peterhuewe@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>