spapr: Clean up DRC set_allocation_state path

The allocation-state indicator should only actually be implemented for
"logical" DRCs, not physical ones.  Factor a check for this, and also for
valid indicator state values into rtas_set_allocation_state().  Because
they don't exist for physical DRCs, there's no reason that we'd ever want
more than one method implementation, so it can just be a plain function.

In addition, the setting to USABLE and setting to UNUSABLE paths in
set_allocation_state() don't actually have much in common.  So, split the
method separate functions for each parameter value (drc_set_usable()
and drc_set_unusable()).

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