sh-pfc: Simplify the sh_pfc_gpio_is_pin() logic

The function is guaranteed to be called with a gpio number smaller than
nr_pins. The condition can the be simplified, and the function inlined.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>