nfp: add MAY_USE_DEVLINK dependency

Fix build with DEVLINK=m and NFP=y.

Fixes: 1851f93f ("nfp: add devlink support")
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>