gpio: pca953x: Make symbol 'pca953x_i2c_regmap' static

Fixes the following sparse warning:

drivers/gpio/gpio-pca953x.c:292:28: warning:
 symbol 'pca953x_i2c_regmap' was not declared. Should it be static?

Fixes: 49427232

 ("gpio: pca953x: Perform basic regmap conversion")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Reviewed-by: Marek Vasut <marex@denx.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>