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(4) accordingly keeping sdhci_generic_tune() a bit cautious
still.

(cherry picked from commit bd15d31cef50060d90356384ba7b878d398fc9f3)