Change how we interpret SDIO device bus speed capability
It appears that not all SDIO devices report a sensible MAX_TRAN_SPEED in their CIS - for example, many Cypress chips report 500 kbit/s there whilst simultaneously reporting support for high-speed bus speed (i.e. 50 MHz) in the CCCR, which makes no sense.
Infer from the wording in the spec (it talks about a minimum value for MAX_TRAN_SPEED for a Low-Speed card, but only of a single value for other cards - one which corresponds to a non-high-speed bus, but this might be a legacy of a realy old version of the spec) that the speed from the CIS is only supposed to apply to Low-Speed cards. There's a flag in the CCCR which indicates such cards.