gpio: pcf857x: Replace 'unsigned' with 'unsigned int'

Cosmetic change only to improve the coding style. No functional change,
since 'unsigned' and 'unsigned int' are identical as far as the compiler
is concerned.

Signed-off-by: Radu Rendec <radu.rendec@gmail.com>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>