pci: return 0 for pci_remap_intr_method MSI-X non-error case

When remapping a MSI-X vector, we would always return ENOENT, even if
successful. This didn't really matter, as the sole caller of
BUS_REMAP_INTR also didn't check for errors.

Return 0 if there's no error, so that we can start handling (or at least
warning about) actual failures.

Reviewed by: jhb
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D41449