[MIPS] au1000: Fix gpio direction

When setting the direction 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.

Signed-off-by: Bruno Randolf <br1@einfach.org>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>