Input: gpio_keys - fix leaking DT node references

for_each_available_child_of_node(node, pp) takes reference to 'pp' and
drops it when attempting next iteration. However if we exit the loop early
we need to drop the reference ourselves.

Tested-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>