st: convert to using driver attr groups for sysfs

st: convert to using driver attr groups for sysfs

This patch changes the st driver to use attribute groups so
driver sysfs files are created automatically. See the
following for reference:

http://kroah.com/log/blog/2013/06/26/how-to-create-a-sysfs-file-correctly/

Signed-off-by: Shane Seymour <shane.seymour@hp.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Kai Mäkisara <kai.makisara@kolumbus.fi>
Signed-off-by: James Bottomley <JBottomley@Odin.com>