powerpc/pseries: Correct MSI allocation tracking

powerpc/pseries: Correct MSI allocation tracking

The per-device MSI allocation calculation in pseries_irq_domain_alloc()
is clearly wrong. It can still happen to work when nr_irqs is 1.

Correct it.

Fixes: c0215e2d72de ("powerpc/pseries: Fix MSI-X allocation failure when quota is exceeded")
Cc: stable@vger.kernel.org
Signed-off-by: Nam Cao <namcao@linutronix.de>
Reviewed-by: Mahesh Salgaonkar <mahesh@linux.ibm.com>
Reviewed-by: Nilay Shroff <nilay@linux.ibm.com>
[maddy: Fixed Nilay's reviewed-by tag]
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20260302003948.1452016-1-namcao@linutronix.de