nfp: correct descriptor offsets in presence of metadata

The TSO-related offsets in the descriptor should not include
the length of the prepended metadata.  Adjust them.  Note that
this could not have caused issues in the past as we don't
support TSO with metadata prepend as of this patch.

Signed-off-by: Michael Rapson <michael.rapson@netronome.com>
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>