mfd: rdc321x: Fix off-by-one for ngpio setting

The valid gpio is GPIO0 ~ GPIO58, so ngpio should be 59.
This patch also renames RDC321X_MAX_GPIO to RDC321X_NUM_GPIO because it
actually means the number of available GPIOs.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>