sfc: mcdi_port: 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/sfc/mcdi_port.c: In function ‘efx_mcdi_phy_decode_link’:
./include/linux/compiler.h:77:22: warning: this statement may fall through [-Wimplicit-fallthrough=]
 # define unlikely(x) __builtin_expect(!!(x), 0)
                      ^~~~~~~~~~~~~~~~~~~~~~~~~~
./include/asm-generic/bug.h:125:2: note: in expansion of macro ‘unlikely’
  unlikely(__ret_warn_on);     \
  ^~~~~~~~
drivers/net/ethernet/sfc/mcdi_port.c:344:3: note: in expansion of macro ‘WARN_ON’
   WARN_ON(1);
   ^~~~~~~
drivers/net/ethernet/sfc/mcdi_port.c:345:2: note: here
  case MC_CMD_FCNTL_OFF:
  ^~~~

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>
Acked-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>