Fix a bug introduced in r327339; at the point in time re-tuning is
executed, the interrupt aggregation code might have disabled the
SDHCI_INT_DMA_END and/or SDHCI_INT_RESPONSE bits in slot->intmask
and the SDHCI_SIGNAL_ENABLE register respectively. So when restoring
the interrupt masks based on the previous contents of slot->intmask
in sdhci_exec_tuning(), ensure that the SDHCI_INT_ENABLE register
doesn't lose these two bits.
While at it and in the spirit of r327339, let sdhci_tuning_intmask()
set the tuning error and re-tuning interrupt bits based on the
SDHCI_TUNING_ENABLED rather than the SDHCI_TUNING_SUPPORTED flag
being set, i. e. only when (re-)tuning is actually used. Currently,
this changes makes no net difference, though.