cnic: Refactor code and mark expected switch fall-through

In preparation to enabling -Wimplicit-fallthrough, refactor code a
bit and mark switch cases where we are expecting to fall through.

This patch fixes the following warning:

drivers/net/ethernet/broadcom/cnic.c: In function ‘cnic_cm_process_kcqe’:
drivers/net/ethernet/broadcom/cnic.c:4044:11: warning: this statement may fall through [-Wimplicit-fallthrough=]
    opcode = L4_KCQE_OPCODE_VALUE_CLOSE_COMP;
drivers/net/ethernet/broadcom/cnic.c:4050:2: note: here
  case L4_KCQE_OPCODE_VALUE_RESET_RECEIVED:
  ^~~~

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>