gpio: zynq: Fixed broken wakeup implementation

Use of unmask/mask in set_wake was an incorrect implementation. The new
implementation correctly sets wakeup for the gpio chip's IRQ so the gpio chip
will not sleep while wakeup-enabled gpio are in use.

Signed-off-by: Ezra Savard <ezra.savard@xilinx.com>
Reviewed-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>