Driver core: Remove unneeded get_{device,driver}() calls.

Code trying to add/remove attributes must hold a reference to
the device resp. driver anyway, so let's remove those reference
count games.
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Cc: Dave Young <hidave.darkstar@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>