[media] rtl2832_sdr: do not use dynamic stack allocation

Do not use dynamic stack allocation.

>> drivers/staging/media/rtl2832u_sdr/rtl2832_sdr.c:181:1:
warning: 'rtl2832_sdr_wr' uses dynamic stack allocation [enabled by default]
Reported-by: NMauro Carvalho Chehab <m.chehab@samsung.com>
Reported-by: Nkbuild test robot <fengguang.wu@intel.com>
Signed-off-by: NAntti Palosaari <crope@iki.fi>
Signed-off-by: NMauro Carvalho Chehab <m.chehab@samsung.com>

drivers/staging/media/rtl2832u_sdr/rtl2832_sdr.c

@@ -156,7 +156,9 @@ static int rtl2832_sdr_wr(struct rtl2832_sdr_state *s, u8 reg, const u8 *val,
 		int len)
 {
 	int ret;
-	u8 buf[1 + len];
+#define MAX_WR_LEN 24
+#define MAX_WR_XFER_LEN (MAX_WR_LEN + 1)
+	u8 buf[MAX_WR_XFER_LEN];
 	struct i2c_msg msg[1] = {
 		{
 			.addr = s->cfg->i2c_addr,
@@ -166,6 +168,9 @@ static int rtl2832_sdr_wr(struct rtl2832_sdr_state *s, u8 reg, const u8 *val,
 		}
 	};
 
+	if (WARN_ON(len > MAX_WR_LEN))
+		return -EINVAL;
+
 	buf[0] = reg;
 	memcpy(&buf[1], val, len);