bnxt: convert to new udp_tunnel_nic infra

Convert to new infra, taking advantage of sleeping in callbacks.

v2:
 - use bp->*_fw_dst_port_id != INVALID_HW_RING_ID as indication
   that the offload is active.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>