nfp: nsp: move default buffer handling into its own function

DMA version of NSP communication is coming, move the code which
copies data into the NFP buffer into a separate function.

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>