dm: blk: Convert interface type to an enum

Since these are sequentially numbered it makes sense to use an enum. It
avoids having to maintain the maximum value, and provides a type we can use
if it is useful.

In fact the maximum value is not used. Rename it to COUNT, since MAX suggests
it is the maximum valid value, but it is not.

Signed-off-by: Simon Glass <sjg@chromium.org>
Tested-by: Stephen Warren <swarren@nvidia.com>