netdevsim: fix fall-through annotation

Replace "pass through" with a proper "fall through" annotation
in order to fix the following warning:

drivers/net/netdevsim/bus.c: In function ‘new_device_store’:
drivers/net/netdevsim/bus.c:170:14: warning: this statement may fall through [-Wimplicit-fallthrough=]
   port_count = 1;
   ~~~~~~~~~~~^~~
drivers/net/netdevsim/bus.c:172:2: note: here
  case 2:
  ^~~~

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

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

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Acked-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>