Staging: et131x: Fix Smatch Warning of Buffer Overflow

This patch fixes the following Sparse Warning in et131x.c-

drivers/staging/et131x/et131x.c:2957 nic_send_packet() error: buffer overflow 'frags' 17 <= 21
drivers/staging/et131x/et131x.c:2959 nic_send_packet() warn: buffer overflow 'frags' 17 <= 21
drivers/staging/et131x/et131x.c:2961 nic_send_packet() error: buffer overflow 'frags' 17 <= 21

Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/et131x/et131x.c

@@ -2902,8 +2902,9 @@ static int nic_send_packet(struct et131x_adapter *adapter, struct tcb *tcb)
 	 * number of fragments. If needed, we can call this function,
 	 * although it is less efficient.
 	 */
-	if (nr_frags > 23)
-		return -EIO;
+
+	/* nr_frags should be no more than 18. */
+	BUILD_BUG_ON(MAX_SKB_FRAGS + 1 > 23);
 
 	memset(desc, 0, sizeof(struct tx_desc) * (nr_frags + 1));