spapr: Clean up RTAS set-indicator

In theory the RTAS set-indicator call can be used for a number of
"indicators" defined by PAPR.  In practice the only ones we're ever likely
to implement are those used for Dynamic Reconfiguration (i.e. hotplug).
Because of this, the current implementation determines the associated DRC
object, before dispatching based on the type of indicator.

However, this means we also need a check that we're dealing with a DR
related indicator at all, which duplicates some of the logic from the
switch further down.

Even though it means a bit of code duplication, things work out cleaner if
we delegate the DRC lookup to the individual indicator type functions -
and it also allows some further cleanups.

While we're there, remove references to "sensor", a copy/paste artefact
from the related, but distinct "get-sensor" call.

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>
