nfp: add devlink_eswitch_mode_set callback

Our previous apps all assumed to use only one eswitch mode (legacy
or switchdev) without the ability to change it.  ABM NIC will
want to support the switch so plumb devlink_eswitch_mode_set through.
The devlink_eswitch_mode_set is expected to spawn representors and
potentially devlink ports so it's called under big devlink lock and
pf->lock.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Dirk van der Merwe <dirk.vandermerwe@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>