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>
diff --git a/cmd/mmc.c b/cmd/mmc.c
index 7bd4cd9..0f1f4e0 100644
--- a/cmd/mmc.c
+++ b/cmd/mmc.c
@@ -331,13 +331,13 @@
 #else
 	original_part = mmc_get_blk_desc(mmc)->hwpart;
 #endif
-	if (blk_select_hwpart_devnum(IF_TYPE_MMC, curr_device, MMC_PART_RPMB) !=
+	if (blk_select_hwpart_devnum(UCLASS_MMC, curr_device, MMC_PART_RPMB) !=
 	    0)
 		return CMD_RET_FAILURE;
 	ret = cp->cmd(cmdtp, flag, argc, argv);
 
 	/* Return to original partition */
-	if (blk_select_hwpart_devnum(IF_TYPE_MMC, curr_device, original_part) !=
+	if (blk_select_hwpart_devnum(UCLASS_MMC, curr_device, original_part) !=
 	    0)
 		return CMD_RET_FAILURE;
 	return ret;
@@ -530,7 +530,7 @@
 	if (!mmc)
 		return CMD_RET_FAILURE;
 
-	mmc_dev = blk_get_devnum_by_type(IF_TYPE_MMC, curr_device);
+	mmc_dev = blk_get_devnum_by_type(UCLASS_MMC, curr_device);
 	if (mmc_dev != NULL && mmc_dev->type != DEV_TYPE_UNKNOWN) {
 		part_print(mmc_dev);
 		return CMD_RET_SUCCESS;
@@ -580,7 +580,7 @@
 	if (!mmc)
 		return CMD_RET_FAILURE;
 
-	ret = blk_select_hwpart_devnum(IF_TYPE_MMC, dev, part);
+	ret = blk_select_hwpart_devnum(UCLASS_MMC, dev, part);
 	printf("switch to partitions #%d, %s\n",
 	       part, (!ret) ? "OK" : "ERROR");
 	if (ret)