[MMC] Ensure correct mmc_priv() behaviour

mmc_priv() has some nasty effects if the wrong pointer type is
passed to it.  Introduce type checking, which also means we get
the right type.  Also add an additional member to mmc_host which
is used to align host-private data appropriately.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>