pseries: Fix some small errors in XICS logic

Under certain circumstances the emulation for the pseries "XICS" interrupt
controller was clearing a pending interrupt from the XISR register, without
also clearing the corresponding priority variable.  This will cause
problems later when can trigger sanity checks in the under-development
in-kernel XICS implementation.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Alexander Graf <agraf@suse.de>
