mcb: Acquire reference to carrier module in core

Acquire a reference to the carrier's kernel module in bus code, so
it can't be removed from the kernel while it still has a bus and thus
possibly devices attached to it.

Signed-off-by: Johannes Thumshirn <jthumshirn@suse.de>
Reported-by: Andreas Werner <andreas.werner@men.de>
Tested-by: Andreas Werner <andreas.werner@men.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>