PCI/MSI: Check for MSI enabled in __pci_msix_enable()

PCI/MSI and PCI/MSI-X are mutually exclusive, but the MSI-X enable code
lacks a check for already enabled MSI.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Ashok Raj <ashok.raj@intel.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://lore.kernel.org/r/20221111122013.653556720@linutronix.de