gpio: xilinx: Simplify logic in xilinx_gpio_set_value

There is no reason to do read/write for if/else separately.

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