[SCSI] sd: convert class code to use dev_groups

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

It required some functions to be moved around to place the show and
store functions next to each other, the old order seemed to make no
sense at all.

Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>