mcb: Introduce type safety for to_mcb_*

The to_mcb_{bus,device,driver}() macros lacked type safety, so convert them to
inline functions to enforce compile time type checking.

Signed-off-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>