pinctrl: use dev_err() to show message in pinctrl_register_one_pin()

Use dev_err() rather than pr_err() to display the error message.
(Besides, dev_err() is already used 7 lines below in this function.)

Also, drop the redundant information "on %s" because dev_err() shows
which device the message is related to.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>