nfp: tls: Fix unreachable code issue

Fix the following unreachable code issue:

   drivers/net/ethernet/netronome/nfp/crypto/tls.c: In function 'nfp_net_tls_add':
   include/linux/compiler_attributes.h:208:41: warning: statement will never be executed [-Wswitch-unreachable]
     208 | # define fallthrough                    __attribute__((__fallthrough__))
         |                                         ^~~~~~~~~~~~~
   drivers/net/ethernet/netronome/nfp/crypto/tls.c:299:3: note: in expansion of macro 'fallthrough'
     299 |   fallthrough;
         |   ^~~~~~~~~~~

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Simon Horman <simon.horman@netronome.com>
Link: https://lore.kernel.org/r/20201117171347.GA27231@embeddedor
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/netronome/nfp/crypto/tls.c

@@ -295,8 +295,8 @@ nfp_net_tls_add(struct net_device *netdev, struct sock *sk,
 			ipv6 = true;
 			break;
 		}
-#endif
 		fallthrough;
+#endif
 	case AF_INET:
 		req_sz = sizeof(struct nfp_crypto_req_add_v4);
 		ipv6 = false;