nfp: separate VXLAN and GRE feature handling

VXLAN and GRE FW features have to currently be both advertised
for the driver to enable them.  Separate the handling.

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>