blk: Rewrite if_type to name functions

These are currently using a simple array lookup in one direction, and
relying on if_type being sequential.

With the move to uclass IDs this needs to change. Update the code to
prepare for the new way. This patch is intended to introduce no
functional change.

The returning of "(none)" from blk_get_if_type_name() is handling a case
that should not happen in either case.

Signed-off-by: Simon Glass <sjg@chromium.org>