cxgb4/cxgb4vf_main: 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/ethernet/chelsio/cxgb4vf/cxgb4vf_main.c: In function ‘fwevtq_handler’:
drivers/net/ethernet/chelsio/cxgb4vf/cxgb4vf_main.c:520:7: warning: this statement may fall through [-Wimplicit-fallthrough=]
   cpl = (void *)p;
   ~~~~^~~~~~~~~~~
drivers/net/ethernet/chelsio/cxgb4vf/cxgb4vf_main.c:524:2: note: here
  case CPL_SGE_EGR_UPDATE: {
  ^~~~

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

Notice that, in this particular case, the code comment is modified
in accordance with what GCC is expecting to find.

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>