nfp: bpf: support BPF offload only on little endian

eBPF is host-endian specific.  Translating both BE and LE eBPF
to the NFP is feasible, but would require quite a bit of indirection.
The fact that I don't have access to any BE hosts that would fit
a 25G/40G/100G NIC is also limiting my ability to test big endian.

For now restrict the offload to little endian hosts only.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>