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>

SVN-Revision: 12561