ACPI: dock: convert sysfs attributes to an attribute_group

As suggested by Dmitry Torokhov, convert the individual sysfs
attributes into an attribute group.

This change eliminates quite a bit of copy/paste code in the
error handling paths.

Signed-off-by: Alex Chiang <achiang@hp.com>
Reviewed-by: Dmitry Torokhov <dtor@mail.ru>
Acked-by: Shaohua Li <shaohua.li@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>