Kickstart should try SDHC mode only when first reading a sector >2G

[gardners]

Trying SDHC mode on SD cards adds ~2 seconds to the kickstart time, instead of SD card access being possible within ~0.2 seconds. This is very annoying. SD mode is okay for SDHC cards when accessing the first 2GB as far as I know. So we should just bring the card up in SD mode, and when first trying to access a sector >2GB, then we should try for SDHC mode, and incur the delay only at that point in time.