pinctrl: sx150x: access the correct bits in the 4-bit regs of sx150[147]

The code assumes 8-bit or 16-bit width registers, but three of the
chips (sx1501/sx1504/sx1507) are 4-bit. So, try to handle 4-bit chips as
well, they leave the high part of each register unused.

Signed-off-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>