gpio: uniphier: fix mismatch between license text and MODULE_LICENSE

gpio: uniphier: fix mismatch between license text and MODULE_LICENSE

The comment block of this file indicates GPL-2.0 "only", while the
MODULE_LICENSE is GPL-2.0 "or later", as include/linux/module.h
describes as follows:

  "GPL"                           [GNU Public License v2 or later]
  "GPL v2"                        [GNU Public License v2]

I am the author of this driver, and my intention is GPL-2.0 "only".

Fixes: dbe776c2ca54 ("gpio: uniphier: add UniPhier GPIO controller driver")
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>