nfp: bpf: rework MTU checking

If control channel MTU is too low to support map operations a warning
will be printed. This is not enough, we want to make sure probe fails
in such scenario, as this would clearly be a faulty configuration.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>