gpio/pca953x: increase variables size to support 24 bit of data

Increase variable size from u16 to u32 to allocate 24 bit of data required for
the TCA6424 I/O expander device type.

Signed-off-by: Leed Aguilar <leed.aguilar@ti.com>
Signed-off-by: Chandrabhanu Mahapatra <cmahapatra@ti.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>