pm80xx: avoid a panic if MSI(X) interrupts are disabled

If MSI(X) interrupts are disabled via the kernel command line
(pci=nomsi), the pm8001 driver will kernel panic because it does not
detect that MSI interrupts are disabled and will soldier on and attempt to
configure MSI interrupts anyways.  This leads to a kernel panic, most
likely because a required data structure is not available down the
line.  Using the pci_msi_enabled() function in order to detect if MSI
interrupts are enabled before configuring them resolves this issue and
avoids a kernel panic when the module is loaded.  Additionally, the
irq_vector structure must be initialized when legacy interrupts are
being used otherwise legacy interrupts will simply not function and
result in another panic.

Signed-off-by: Benjamin Rood <brood@attotech.com>
Reviewed-by: Jack Wang <jinpu.wang@profitbricks.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>