mmc: Use bus_topo_lock and taskqueue_bus while adding/removing child devices

[jhb]

Summary

Some drivers held regular mutexes across some new-bus calls, instead
depend on bus_topo_lock to protect the relevant softc members. This
also fixes the bus_topo_lock to be explicit in these drivers rather
than relying on the implicit Giant from taskqueue_swi_giant, and it
avoids calling sleepable routines like device_probe_and_attach from an
swi context.