bnxt_en: Refactor filter insertion logic in bnxt_rx_flow_steer().

Add a new function bnxt_insert_ntp_filter() to insert the ntuple filter
into the hash table and other basic setup.  We'll use this function
to insert a user defined filter from ethtool.

Also, export bnxt_lookup_ntp_filter_from_idx() and bnxt_get_ntp_filter_idx()
for similar purposes.  All ntuple related functions are now no longer
compiled only for CONFIG_RFS_ACCEL

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>
Signed-off-by: David S. Miller <davem@davemloft.net>