ppc/xive: Explicitly zero NSR after accepting

ppc/xive: Explicitly zero NSR after accepting

Have xive_tctx_accept clear NSR in one shot rather than masking out bits
as they are tested, which makes it clear it's reset to 0, and does not
have a partial NSR value in the register.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: Glenn Miles <milesg@linux.ibm.com>
Reviewed-by: Michael Kowal <kowal@linux.ibm.com>
Reviewed-by: Caleb Schlossin <calebs@linux.ibm.com>
Tested-by: Gautam Menghani <gautam@linux.ibm.com>
Link: https://lore.kernel.org/qemu-devel/20250512031100.439842-15-npiggin@gmail.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>