isci: check return value of pci_alloc_msix()

Certain VM guest types (VMware, Xen) do not support MSI, so pci_alloc_msix()
always fails. isci(4) was not properly detecting the allocation failure,
and would try to proceed with MSIx resource initialization rather than
reverting to INTx.

Reported and tested by: Bradley W. Dutton (brad-fbsd-stable@duttonbros.com)
MFC after: 3 days
Sponsored by: Intel