pseries: Fix semantics of RTAS int-on, int-off and set-xive functions

pseries: Fix semantics of RTAS int-on, int-off and set-xive functions

Currently the ibm,int-on and ibm,int-off RTAS functions are implemented as
no-ops.  This is because when implemented as specified in PAPR they caused
Linux (which calls both int-on/off and set-xive) to end up with interrupts
masked when they should not be.  Since Linux's set-xive calls make the
int-on/off calls redundant, making them nops worked around the problem.

In fact, the problem was caused because there was a subtle bug in set-xive,
PAPR specifies that as well as updating the current priority, it also needs
to update the saved priority used by int-on/off.  With this bug fixed the
problem goes away.  This patch implements this more correct fix.

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