MMC compatibility fix

The following three patches resolve the issues with some MMC cards failing to work with the current MMC layer.

Some of the issues are because of cards failing to comply with standards. But since we cannot fix the cards we have to deal with the problems on our side.

All three patches solve initialisation problems:

- Standard requires a GO_IDLE before changing the OCR mask.

- Standard doesn't define how long to wait for cards to power up. The current time as been shown to be too short.

- Empty OCR mask causes some cards to fail (even though an empty OCR is ok according to the standard).

--
Pierre Ossman