mmc: block: Increase max_devices

Currently the driver imposes a limit of 256 total minor numbers,
apparently based on the historic Unix/Linux limit.  This is quite
restrictive, particularly if we raise the maximum number of
partitions per card to 256 to match sd.

In order to make the full minor number space available we would
have to replace the static dev_use and name_use arrays with struct
ida.  But we can at least allow use of 256 cards rather than just
256 minors, with only a small change.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>