nfp: silence set but not used warning with IPV6=n

nfp: silence set but not used warning with IPV6=n

Test robot reports:

drivers/net/ethernet/netronome/nfp/crypto/tls.c: In function 'nfp_net_tls_rx_resync_req':
drivers/net/ethernet/netronome/nfp/crypto/tls.c:477:18: warning: variable 'ipv6h' set but not used [-Wunused-but-set-variable]
  477 |  struct ipv6hdr *ipv6h;
      |                  ^~~~~
In file included from include/linux/compiler_types.h:65,
                    from <command-line>:
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;
      |   ^~~~~~~~~~~

Use the IPv6 header in the switch, it doesn't matter which header
we use to read the version field.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>