spapr: Clean up DR entity sense handling

spapr: Clean up DR entity sense handling

DRC classes have an entity_sense method to determine (in a specific PAPR
sense) the presence or absence of a device plugged into a DRC.  However,
we only have one implementation of the method, which explicitly tests for
different DRC types.  This changes it to instead have different method
implementations for the two cases: "logical" and "physical" DRCs.

While we're at it, the entity sense method always returns RTAS_OUT_SUCCESS,
and the interesting value is returned via pass-by-reference.  Simplify this
to directly return the value we care about

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>