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 <gustavo@embeddedor.com>
Signed-off-by: David S. Miller <davem@davemloft.net>