gpio: brcmstb: Use list_for_each_entry at appropriate places

Use list_for_each_entry to simplify the code a bit.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Gregory Fong <gregory.0xf0@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>