gpio-intel-mid: update prefixes and names from langwell to intel-mid

After file was renamed from gpio-langwell to gpio-intel-mid, this patch
updates the variables, functions and structs to be based on intel-mid
instead of langwell.

There is no function change.

Signed-off-by: David Cohen <david.a.cohen@linux.intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>