nfp: prepare for more TX metadata prepend

Subsequent patches will add support for more TX metadata fields.
Prepare for this by handling an additional double word - firmware
handle as metadata type 7.

Signed-off-by: Dirk van der Merwe <dirk.vandermerwe@netronome.com>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>