pinctrl: sh-pfc: gpio: Return early in gpio_pin_to_irq()

As of commit 4adeabd0 ("pinctrl: sh-pfc: Remove hardcoded IRQ
numbers"), only a single operation needs to be performed after finding
the wanted pin.  Hence decrease the needed attention span of the casual
reader by replacing the goto by a direct return.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Link: https://lore.kernel.org/r/20200212090200.11106-1-geert+renesas@glider.be