rfkill: rfkill-hks: Use devm_fwnode_get_gpiod_from_child

devm_fwnode_gpiod_get does not exist in 5.3. We keep this as a separate
patch so the code stays close to 5.6 otherwise.

Signed-off-by: Guido Günther <guido.gunther@puri.sm>
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