gpio: fix warning about iterator

We were getting build warning about "iterator" being used uninitialized.
Use iterator properly to fix the build warning and in the process remove
the variable "pos" which is not required now.

Signed-off-by: Sudip Mukherjee <sudip@vectorindia.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>