sh: fix DMAOR register access on SH7709

sh7709 hardware manual says DMAOR is 16 bits long on this platform.

Tested and working with a modified smsc911x ethernet driver (sh-dma
support patch for this driver is coming soon).

Signed-off-by: Steve Glendinning <steve.glendinning@smsc.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>