fix(mmc): do not modify r_data in mmc_send_cmd()

Fix MISRA C2012-17.8:
A function parameter should not be modified

Signed-off-by: Yann Gautier <yann.gautier@st.com>
Change-Id: I08ed4c6768c7988a98de21da181b97b0885139de
diff --git a/drivers/mmc/mmc.c b/drivers/mmc/mmc.c
index 1a63f61..d6f4421 100644
--- a/drivers/mmc/mmc.c
+++ b/drivers/mmc/mmc.c
@@ -69,8 +69,7 @@
 		int i;
 
 		for (i = 0; i < 4; i++) {
-			*r_data = cmd.resp_data[i];
-			r_data++;
+			r_data[i] = cmd.resp_data[i];
 		}
 	}