PCI: vmd: Remove MSI-X check on child devices

d7d8ab87 ("PCI: vmd: Switch to msi_create_parent_irq_domain()") added a
WARN_ON sanity check that child devices support MSI-X, because VMD document
says [1]:

  Intel VMD only supports MSIx Interrupts from child devices and therefore
  the BIOS must enable PCIe Hot Plug and MSIx interrups [sic].

However, the VMD device can't even tell the difference between a child
device using MSI and one using MSI-X.  Per 185a383a ("x86/PCI: Add
driver for Intel Volume Management Device (VMD)"), VMD does not support
INTx interrupts, but does support child devices using either MSI or MSI-X.

Remove the sanity check to avoid the WARN_ON and allow child devices to use
MSI, reported by Ammar.

Fixes: d7d8ab87 ("PCI: vmd: Switch to msi_create_parent_irq_domain()")
Link: https://cdrdv2-public.intel.com/776857/VMD_White_Paper.pdf

 [1]
Reported-by: Ammar Faizi <ammarfaizi2@gnuweeb.org>
Closes: https://lore.kernel.org/linux-pci/aJXYhfc%2F6DfcqfqF@linux.gnuweeb.org/


Signed-off-by: Nam Cao <namcao@linutronix.de>
[bhelgaas: commit log]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Tested-by: Ammar Faizi <ammarfaizi2@gnuweeb.org>
Link: https://patch.msgid.link/20250811053935.4049211-1-namcao@linutronix.de