pinctrl: renesas: checker: Simplify same_name()

Simplify the helper that checks if two strings are valid and identical.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/509c70041bcf4302e284758abe1fd8165644b505.1640270559.git.geert+renesas@glider.be