nfp: bpf: add support for XDP_FLAGS_HW_MODE

Respect the XDP_FLAGS_HW_MODE.  When it's set install the program
on the NIC and skip enabling XDP in the driver.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>