nfp: support variable NSP response lengths

We want to support extendable commands, where newer versions
of the management FW may provide more information.  Zero out
the communication buffer before passing control to NSP.  This
way if management FW is old and only fills in first N bytes,
the remaining ones will be zeros which extended ABI fields
should reserve as not supported/not available.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>