nfp: xdp: move driver XDP setup into a separate function

In preparation of XDP offload flags move the driver setup into
a function.  Otherwise the number of conditions in one function
would make it slightly hard to follow.  The offload handler may
now be called with NULL prog, even if no offload is currently
active, but that's fine, offload code can handle that.

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