pinctrl: rockchip: Simplify pin_to_bank equation

If (b->pin_base + b->nr_pins - 1) < pin is true, pin >= b->pin_base is always
true because b->nr_pins is never less than 0.
Thus this patch simplify the equation.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>