pinctrl: do not care about blank pin name

If a pin name is not specified in struct pinctrl_pin_desc,
pinctrl_register_one_pin() dynamically assigns its name.
So, desc->name is always a valid pointer here.

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