spapr: Refactor spapr_drc_detach()

This function has two unused parameters - remove them.

It also sets awaiting_release on all paths, except one.  On that path
setting it is harmless, since it will be immediately cleared by
spapr_drc_release().  So factor it out of the if statements.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Greg Kurz <groug@kaod.org>
Tested-by: Daniel Barboza <danielhb@linux.vnet.ibm.com>