Add locking to mcd(4) and mark MPSAFE.

Add locking to mcd(4) and mark MPSAFE.
- Actually use existing per-softc mutex.
- Use mutex in cdev routines and remove D_NEEDGIANT.
- Use callout(9) instead of timeout(9).
- Don't check for impossible conditions (e.g. MCDINIT being clear).
- Remove critical_enter/exit when sending a PIO command.
- Use bus_*() instead of bus_space_*().

Tested by:	no one