nfp: encapsulate NSP command arguments into structs

There is already a fair number of arguments to nfp_nsp_command()
family of functions.  Encapsulate them into structures to make
adding new ones easier.  No functional changes.
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Dirk van der Merwe <dirk.vandermerwe@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>