samples: hid-mouse: various cleanups

Use the new GPIO_DT_SPEC_GET_OR() macro to clean up the node detection
code by eliminating conditional compilation. This increases code
coverage on platforms where not all the devicetree nodes are
available.

Clean up some whitespace while we're at it.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>