mmc_cmds.c: fix build with gcc 4.9

Fix following error with gcc 4.9:

mmc_cmds.c:1918:9: error: missing braces around initializer [-Werror=missing-braces]
  struct rpmb_frame frame_status = {0};

Fixes:
 - http://autobuild.buildroot.org/results/bf3b6f9f6ef39b99842b3c92495b7bf359c68158

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Link: https://lore.kernel.org/r/20190928185107.21125-1-fontaine.fabrice@gmail.com

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>

mmc_cmds.c

@@ -1982,7 +1982,9 @@ static int do_rpmb_op(int fd,
 	u_int16_t rpmb_type;
 	struct mmc_ioc_multi_cmd *mioc;
 	struct mmc_ioc_cmd *ioc;
-	struct rpmb_frame frame_status = {0};
+	struct rpmb_frame frame_status;
+
+	memset(&frame_status, 0, sizeof(frame_status));
 
 	if (!frame_in || !frame_out || !out_cnt)
 		return -EINVAL;