gpio: pca953x: fix gpio input on gpio offsets >= 8

This change fixes a regression introduced by commit
f5f0b7aa (gpio: pca953x: make the register access by GPIO bank)

When the pca953x driver was converted to using 8-bit reads/writes
the bitmask in pca953x_gpio_get_value wasn't adjusted with a
modulus BANK_SZ and consequently looks at the wrong bits in the
input register.

Signed-off-by: Andrew Ruder <andrew.ruder@elecsyscorp.com>
Reviewed-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>