powerpc/xive: Fix IPI reset

When resetting an IPI, hw_ipi should also be set to zero.

Fixes: eac1e731

 ("powerpc/xive: guest exploitation of the XIVE interrupt controller")
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>