MFC: r270885, r270948

- Nuke unused sdhci_softc.
- Static'ize sdhci_debug local to sdhci.c.
- Const'ify PCI device description strings.
- Nuke redundant resource ID members from sdhci_pci_softc.
- Nuke unused hw.sdhci_pci.debug tunable.
- Add support for using MSI instead of INTx, controllable via the tunable
  hw.sdhci.enable_msi (defaulting to on) and tested with a RICOH R5CE823 SD
  controller.
- Use NULL instead of 0 for pointers.