- Simplify MSI allocation to what is actually needed for a single one.
- Release the MSI and the corresponding bus resource as appropriate when
either the interrupt resource cannot be allocated or setting up the
interrupt fails.
- Error out when interrupt allocation or setup fails and polling is
disabled.
- Release the MSI after the corresponding bus resource so the former is
not leaked on detach.
- Remove a redundant softc member.
MFC after: 3 days
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