MEI: convert bus code to use dev_groups

The dev_attrs field of struct bus_type is going away soon, dev_groups
should be used instead.  This converts the MEI bus code to use
the correct field.

Acked-by: Tomas Winkler <tomas.winkler@intel.com>
Cc: Samuel Ortiz <sameo@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>