nfp: bpf: add short busy wait for FW replies

Scheduling out and in for every FW message can slow us down
unnecessarily.  Our experiments show that even under heavy load
the FW responds to 99.9% messages within 200 us.  Add a short
busy wait before entering the wait queue.

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