tools: bpftool: use 4 context mode for the NFP disasm

The nfp driver is currently always JITing the BPF for 4 context/thread
mode of the NFP flow processors.  Tell this to the disassembler,
otherwise some registers may be incorrectly decoded.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Jiong Wang <jiong.wang@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>