Some MMC hosts can only handle log2 block sizes.  Unfortunately,
the MMC password support needs to be able to send non-log2 block
sizes.  Provide a capability so that the MMC password support can
decide whether it should use this support or not.

The unfortunate side effect of this host limitation is that any
MMC card protected by a password which is not a log2 block size
can not be accessed on a host which only allows a log2 block size.

This change just adds the flag.  The MMC password support code
needs updating to use it (if and when it is finally submitted.)
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>