MFC: r327339, r327924

MFC: r327339, r327924

- There is no need to keep the tuning error and re-tuning interrupts
  enabled (though, no interrupt generation enabled for them) all the
  time as soon as (re-)tuning is supported; only enable them and let
  them generate interrupts when actually using (re-)tuning.
- Also disable all interrupts except SDHCI_INT_DATA_AVAIL ones while
  executing tuning and not just their signaling.
- Set the tuning error and re-tuning interrupt enable bits based on
  the SDHCI_TUNING_ENABLED rather than the SDHCI_TUNING_SUPPORTED flag,
  i. e. only when (re-)tuning is actually used. Currently, this change
  makes no net difference, though.

git-svn-id: svn://svn.freebsd.org/base/stable/10@331035 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f