gpio: xilinx: Not read output values via regs

Reading registers for finding out output value is not working because
input value is read instead in case of tristate.

Reported-by: Stefan Herbrechtsmeier <stefan@herbrechtsmeier.net>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Stefan Herbrechtsmeier <stefan@herbrechtsmeier.net>