button: make button_get_by_label() case insensitive

button: make button_get_by_label() case insensitive

This function is already doing a fuzzy match, since there are no
guarantees that a given label is unique.

Ignoring case makes it much easier to catch "Volume down" or "Volume
Down" in board-agnostic code.

Tested-by: Danila Tikhonov <danila@jiaxyga.com> # google-sunfish
Tested-by: Jens Reidel <adrian@mainlining.org> # xiaomi-davinci
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://lore.kernel.org/r/20250331-qcom-phones-v4-6-f52e57d3b8c6@linaro.org
Signed-off-by: Caleb Connolly <caleb.connolly@linaro.org>