au1000: fix gpio output

when setting the output state of one GPIO pin we have to keep the state of the
other pins, hence use binary OR. also gpio_direction_output() wants to set an
initial value, so add that too.

this fixes a problem with the USB power switch on mtx-1 boards. (#3974)

Signed-off-by: Bruno Randolf <br1@einfach.org>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@12561 3c298f89-4303-0410-b956-a3cf2f4a3e73