gpio-uclass: fix gpio lookup by label

gpio-uclass: fix gpio lookup by label

Matching anything that just happens to have the sought-for label as a
prefix is wrong. For example, if the board designer has designated 10
lines for debug purposes, named "debug1" through "debug10", and we are
looking up "debug1", if debug10 happens to be met first during the
iteration we'd wrongly return that.

In theory, this can break existing users that could rely on this
quirk, but OTOH keeping the current broken semantics can cause a lot
of grief for people hitting this in the future and not understanding
why they don't find the line they expect. Considering how few in-tree
defconfigs currently set DM_GPIO_LOOKUP_LABEL (ignoring sandbox, only
four "real" boards), let's fix it before the use becomes more
widespread.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Heiko Schocher <hs@denx.de>