gpio: ath79: Check valid gpio count for both DT and non-DT cases

Move the code checking valid gpio count to cover both DT and non-DT cases.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Alban Bedel <albeu@free.fr>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>