gpio: ws16c48: Remove unnecessary driver_data set

Setting driver_data was necessary to access private data in the
ws16c48_remove function. Now that the ws16c48_remove function is gone,
driver_data is no longer used. This patch removes the relevant code.

Signed-off-by: William Breathitt Gray <vilhelm.gray@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>