net: usb: pegasus: Mark expected switch fall-through

In preparation to enabling -Wimplicit-fallthrough, mark switch
cases where we are expecting to fall through.

Warning level 3 was used: -Wimplicit-fallthrough=3

This patch is part of the ongoing efforts to enabling
-Wimplicit-fallthrough.
Signed-off-by: NGustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/usb/pegasus.c

@@ -1011,6 +1011,7 @@ static int pegasus_ioctl(struct net_device *net, struct ifreq *rq, int cmd)
 	switch (cmd) {
 	case SIOCDEVPRIVATE:
 		data[0] = pegasus->phy;
+		/* fall through */
 	case SIOCDEVPRIVATE + 1:
 		read_mii_word(pegasus, data[0], data[1] & 0x1f, &data[3]);
 		res = 0;