net: dsa: microchip: mark expected switch fall-through

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

This patch fixes the following warning:

drivers/net/dsa/microchip/ksz9477.c: In function ‘ksz9477_get_interface’:
drivers/net/dsa/microchip/ksz9477.c:1145:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
if (gbit)
^
drivers/net/dsa/microchip/ksz9477.c:1147:2: note: here
case 0:
^~~~

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

This patch is part of the ongoing efforts to enable
-Wimplicit-fallthrough.

Signed-off-by: Gustavo A. R. Silva <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/net/dsa/microchip/ksz9477.c

···
 		interface = PHY_INTERFACE_MODE_GMII;
 		if (gbit)
 			break;
+		/* fall through */
 	case 0:
 		interface = PHY_INTERFACE_MODE_MII;
 		break;