mmc: add support for write protection

Add generic mmc write protection functionality.

Signed-off-by: Nikita Kiryanov <nikita@compulab.co.il>
Signed-off-by: Igor Grinberg <grinberg@compulab.co.il>
diff --git a/common/cmd_mmc.c b/common/cmd_mmc.c
index 7dacd51..8c53a10 100644
--- a/common/cmd_mmc.c
+++ b/common/cmd_mmc.c
@@ -282,6 +282,13 @@
 
 		mmc_init(mmc);
 
+		if ((state == MMC_WRITE || state == MMC_ERASE)) {
+			if (mmc_getwp(mmc) == 1) {
+				printf("Error: card is write protected!\n");
+				return 1;
+			}
+		}
+
 		switch (state) {
 		case MMC_READ:
 			n = mmc->block_dev.block_read(curr_device, blk,