pinctrl: sh-pfc: Correct printk level of group reference warning

Improve wording while at it.

Fixes: 6161b39a ("pinctrl: sh-pfc: Validate pinmux tables at runtime when debugging")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>