spapr: Change DRC attach & detach methods to functions

spapr: Change DRC attach & detach methods to functions

DRC objects have attach & detach methods, but there's only one
implementation.  Although there are some differences in its behaviour for
different DRC types, the overall structure is the same, so while we might
want different method implementations for some parts, we're unlikely to
want them for the top-level functions.

So, replace them with direct function calls.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Acked-by: Michael Roth <mdroth@linux.vnet.ibm.com>