pinctrl: sh-pfc: Use dev_notice_once() instead of open-coding

At the time of commit 9a643c9a ("sh-pfc: Convert message
printing from pr_* to dev_*"), the dev_*_once() variants didn't exist
yet, so the once behavior was open-coded.

Since commit e135303b ("device: Add dev_<level>_once variants")
they do, so "revert" to the good practice of using a helper.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Ulrich Hecht <uli+renesas@fpond.eu>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>