nfp: tls: don't leave key material in freed FW cmsg skbs

Make sure the contents of the skb which carried key material
to the FW is cleared.

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>