powerpc/xive: Fix error handling when allocating an IPI

powerpc/xive: Fix error handling when allocating an IPI

This is a smatch warning:

  arch/powerpc/sysdev/xive/common.c:1161 xive_request_ipi() warn: unsigned 'xid->irq' is never less than zero.

Fixes: fd6db2892eba ("powerpc/xive: Modernize XIVE-IPI domain with an 'alloc' handler")
Cc: stable@vger.kernel.org # v5.13
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210701152412.1507612-1-clg@kaod.org