pinctrl: sx150x: handle missing 'advanced' reg in sx1504 and sx1505

This fixes a problem where sx150x_regmap_reg_width() returns 8 for the
data register (reg 0) for sx1504 where it should return 4, and return
a correct 8 for sx1505 but for the wrong reason (both chips lack the
'advanced' register). This is not a real problem, since nothing depends
on the function returning 4 or 8, and certainly not if it is returning
8 for the wrong reason. But fix this to avoid nasty surprises down the
line.

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