bnxt_en: Refactor bnxt_ntuple_filter structure.

This is in preparation to support user defined L2 (ether) filters,
which will have many similarities with ntuple filters.  Refactor
bnxt_ntuple_filter structure to have a bnxt_filter_base structure
that can be re-used by the L2 filters.

Reviewed-by: Vasundhara Volam <vasundhara-v.volam@broadcom.com>
Reviewed-by: Andy Gospodarek <andrew.gospodarek@broadcom.com>
Reviewed-by: Pavan Chebbi <pavan.chebbi@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>