dm: env: mmc: Convert env_mmc to support CONFIG_BLK

Update the MMC environment code so that it works with driver-model enabled
for block devices.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/common/env_mmc.c b/common/env_mmc.c
index c7fef18..16f6a17 100644
--- a/common/env_mmc.c
+++ b/common/env_mmc.c
@@ -128,12 +128,12 @@
 			    unsigned long offset, const void *buffer)
 {
 	uint blk_start, blk_cnt, n;
+	struct blk_desc *desc = mmc_get_blk_desc(mmc);
 
 	blk_start	= ALIGN(offset, mmc->write_bl_len) / mmc->write_bl_len;
 	blk_cnt		= ALIGN(size, mmc->write_bl_len) / mmc->write_bl_len;
 
-	n = mmc->block_dev.block_write(&mmc->block_dev, blk_start,
-					blk_cnt, (u_char *)buffer);
+	n = blk_dwrite(desc, blk_start, blk_cnt, (u_char *)buffer);
 
 	return (n == blk_cnt) ? 0 : -1;
 }
@@ -197,12 +197,12 @@
 			   unsigned long offset, const void *buffer)
 {
 	uint blk_start, blk_cnt, n;
+	struct blk_desc *desc = mmc_get_blk_desc(mmc);
 
 	blk_start	= ALIGN(offset, mmc->read_bl_len) / mmc->read_bl_len;
 	blk_cnt		= ALIGN(size, mmc->read_bl_len) / mmc->read_bl_len;
 
-	n = mmc->block_dev.block_read(&mmc->block_dev, blk_start, blk_cnt,
-				      (uchar *)buffer);
+	n = blk_dread(desc, blk_start, blk_cnt, (uchar *)buffer);
 
 	return (n == blk_cnt) ? 0 : -1;
 }