nfp: fix old kdoc issues

Since commit 3a025e1d

 ("Add optional check for bad kernel-doc
comments") when built with W=1 build will complain about kdoc errors.
Fix the kdoc issues we have.  kdoc is still confused by defines in
nfp_net_ctrl.h but those are not really errors.

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