net/af_xdp: allow probing when multiprocess is disabled

net/af_xdp: allow probing when multiprocess is disabled

If EAL multiprocess feature has been disabled via rte_mp_disable()
function, AF_XDP driver may not be able to register its IPC callback.
Previously this leads to probe failure.
This commit adds a check for this condition so that AF_XDP can still be
used even if multiprocess is disabled.

Fixes: 9876cf8316b3 ("net/af_xdp: re-enable secondary process support")
Signed-off-by: Junxiao Shi <git@mail1.yoursunny.com>
Acked-by: Ciara Loftus <ciara.loftus@intel.com>