gpio: just plain warning when nonexisting gpio requested

gpio: just plain warning when nonexisting gpio requested

When trying to export an nonexisting gpio ID, the kernel prints out a
big warning w/ stacktrace, sounding like a huge problem. In fact it's
a pretty normal situation, like file or device not found.

So, just print a more relaxed warning instead.

changes v2: drop defining pr_fmt()

Signed-off-by: Enrico Weigelt <info@metux.net>
Link: https://lore.kernel.org/r/20201202133754.32045-1-info@metux.net
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>