refactor(mmc): replace magic value with new PART_CFG_BOOT_PARTITION_NO_ACCESS

Disabling access to the boot partition reverts the MMC to read from the
user area. Add a macro to make this clearer.

Suggested-by: Manish V Badarkhe <Manish.Badarkhe@arm.com>
Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Change-Id: I34a5a987980bb4690d08d255f465b11a4697ed5a
diff --git a/drivers/mmc/mmc.c b/drivers/mmc/mmc.c
index 4917f02..0458733 100644
--- a/drivers/mmc/mmc.c
+++ b/drivers/mmc/mmc.c
@@ -778,7 +778,7 @@
 {
 	int ret;
 
-	ret = mmc_part_switch(0);
+	ret = mmc_part_switch(PART_CFG_BOOT_PARTITION_NO_ACCESS);
 	if (ret < 0) {
 		ERROR("Failed to switch to user partition, %d\n", ret);
 	}