Make subr_bus.c more consistent with regard to style(9) and itself.

Make subr_bus.c more consistent with regard to style(9) and itself.

- adjust the 4 space indentation in the oldest parts
- use return(value) form
- move returns into the default case of certain switch statements
- make some pointer checks explicit against NULL
- reorder device_probe_and_attach to simplify the if's
- remove unnecessary return; at the end of functions