MIPS: RB532: Auto disable GPIO alternate function

When a driver calls gpio_set_direction_{input,output}(), it obviously
doesn't want the alternate function for that pin to be active (as the
direction would not matter in that case). This patch ensures alternate
function is disabled when the direction is being changed.

Signed-off-by: Phil Sutter <n0-1@freewrt.org>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>