nfp: devlink add support for getting eswitch mode

Add app callback for reporting eswitch mode.  Non-SRIOV apps
should not implement this callback, nfp_app code will then
respond with -EOPNOTSUPP.

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