Since r273264 the SD card detection on Raspberry Pi is reliably working and
that expose new bugs with HS mode.

When the old code could not do the proper card detection it would boot with
lower defaults (and no HS mode) and this makes some HS cards boots.

Now, with the card always identified as HS capable, the sdhci controller
tries to run the card at HS speeds and makes the boot always fail.

Disable the HS mode for now (which still can be enabled with the tunable)
until it is properly fixed.

MFC with:	r273264
Requested by:	many