gpio: pca953x: avoid to use uninitialized value pinctrl

There is a variable pinctrl declared without initializer. And then
has the case (switch operation chose the default case) to directly
use this uninitialized value, this is not a safe behavior. So here
initialize the pinctrl as 0 to avoid this issue.
This is reported by Coverity.

Fixes: 13c5d4ce ("gpio: pca953x: Add support for PCAL6534")
Signed-off-by: Haibo Chen <haibo.chen@nxp.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>