Allow uart(4) to use MSI interrupts on single-port PCI instances.

Do this here as puc(4) disallows single-port instances; at least
one multi-port PCIe UART chip (in this case, the ASIX MCS9922)
present separate PCI configuration space (functions) for each UART.

Tested using lrzsz and a null-modem cable. The ExpressCard/34
variants containing the MCS9922 should also use MSI with this change.

Reviewed by: jhb, imp, rpokala
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D9123