ppc/xive: tctx_accept only lower irq line if an interrupt was presented

ppc/xive: tctx_accept only lower irq line if an interrupt was presented

The relationship between an interrupt signaled in the TIMA and the QEMU
irq line to the processor to be 1:1, so they should be raised and
lowered together and "just in case" lowering should be avoided (it could
mask

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: Glenn Miles <milesg@linux.ibm.com>
Tested-by: Gautam Menghani <gautam@linux.ibm.com>
Link: https://lore.kernel.org/qemu-devel/20250512031100.439842-35-npiggin@gmail.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>