MIPS: ath79: fix ar933x uart parity mode

While trying to get the uart with parity working I found setting even
parity enabled odd parity insted. Fix the register settings to match
the datasheet of AR9331.

A similar patch was created by 8devices, but not sent upstream.
https://github.com/8devices/openwrt-8devices/commit/77c5586ade3bb72cda010afad3f209ed0c98ea7c



Signed-off-by: Stefan Hellermann <stefan@the2masters.de>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: linux-mips@vger.kernel.org