mmc: Fixes for Dual Data Rate (DDR) support

The DDR support patch needs the following fixes:

- The block driver does not need to know about DDR, any more
  than it needs to know about bus width.
- Not only the card must be switched to DDR mode.  The host
  controller must also be configured, which is done through
  the 'set_ios()' function.
- Do not set the DDR mode state until after the switch command
  is successful.
- Setting block length is not supported in DDR mode.  Make that
  a core function and change the other place it is used (mmc_test)
  also.

Signed-off-by: Adrian Hunter <adrian.hunter@nokia.com>
Signed-off-by: Chris Ball <cjb@laptop.org>