mips: convert vpe_class to use dev_groups

The dev_attrs field of struct class is going away soon, dev_groups
should be used instead.  This converts the MIPS vpe_class code to use
the correct field.

Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: John Crispin <blogic@openwrt.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>