mmc(4): Don't call bridge driver for timings not requiring tuning

The original idea behind calling into the bridge driver was to have the
logic deciding whether tuning is actually required for a particular bus
timing in a given slot as well as doing the sanity checking only on the
controller layer which also generally is better suited for these due to
say SDHCI_SDR50_NEEDS_TUNING. On another thought, not every such driver
should need to check whether tuning is required at all, though, and not
everything is SDHCI in the first place.
Adjust sdhci{,_fsl_fdt}(4) accordingly, but keep sdhci_generic_tune() a
bit cautious still.