gpio: pca953x: Fix pca953x_gpio_set_config

pca953x_gpio_set_config is setup to support pull-up/down
bias.  Currently the driver uses a variable called 'config' to
determine which options to use.  Unfortunately, this is incorrect.

This patch uses function pinconf_to_config_param(config), which
converts this 'config' parameter back to pinconfig to determine
which option to use.

Fixes: 15add068 ("gpio: pca953x: add ->set_config implementation")
Signed-off-by: Adam Ford <aford173@gmail.com>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>