app/testpmd: fix build on FreeBSD

app/testpmd: fix build on FreeBSD

Build log:
/root/dpdk/app/test-pmd/cmdline.c:6687:45: error: no member named
's6_addr32' in 'struct in6_addr'
    rte_be_to_cpu_32(res->ip_value.addr.ipv6.s6_addr32[i]);

This is caused by macro "s6_addr32" not defined on FreeBSD and testpmd
swap big endian parameter to host endian. Move the swap action to i40e
ethdev will fix this issue.

Fixes: 7b1312891b69 ("ethdev: add IP in GRE tunnel")

Signed-off-by: Marvin Liu <yong.liu@intel.com>
Acked-by: Jingjing Wu <jingjing.wu@intel.com>
Tested-by: Bruce Richardson <bruce.richardson@intel.com>