blk: Switch over to using uclass IDs

We currently have an if_type (interface type) and a uclass id. These are
closely related and we don't need to have both.

Drop the if_type values and use the uclass ones instead.

Maintain the existing, subtle, one-way conversion between UCLASS_USB and
UCLASS_MASS_STORAGE for now, and add a comment.

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

arch/arm/mach-stm32mp/cmd_stm32prog/stm32prog.c

@@ -1823,7 +1823,7 @@ static int part_delete(struct stm32prog_data *data,
 		 * need to switch to associated hwpart 1 or 2
 		 */
 		if (part->part_id < 0)
-			if (blk_select_hwpart_devnum(IF_TYPE_MMC,
+			if (blk_select_hwpart_devnum(UCLASS_MMC,
 						     part->dev->dev_id,
 						     -part->part_id))
 				return -1;
@@ -1832,7 +1832,7 @@ static int part_delete(struct stm32prog_data *data,
 
 		/* return to user partition */
 		if (part->part_id < 0)
-			blk_select_hwpart_devnum(IF_TYPE_MMC,
+			blk_select_hwpart_devnum(UCLASS_MMC,
 						 part->dev->dev_id, 0);
 		if (blks != blks_size) {
 			ret = -1;