gpio: xilinx: Fix bug where the wrong GPIO register is written to

gpio: xilinx: Fix bug where the wrong GPIO register is written to

Care is taken with "index", however with the current version
the actual xgpio_writereg is using index for data but
xgpio_regoffset(chip, i) for the offset. And since i is already
incremented it is incorrect. This patch fixes it so that index
is used for the offset too.

Cc: stable@vger.kernel.org
Signed-off-by: Paul Thomas <pthomas8589@gmail.com>
Link: https://lore.kernel.org/r/20200125221410.8022-1-pthomas8589@gmail.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>