dma: convert dma_devclass to use dev_groups

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

Cc: Dan Williams <djbw@fb.com>
Acked-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>