nfp: make sure representors are destroyed before their lower netdev

App start/stop callbacks can perform application initialization.
Unfortunately, flower app started using them for creating and
destroying representors.  This can lead to a situation where
lower vNIC netdev is destroyed while representors still try
to pass traffic.  This will most likely lead to a NULL-dereference
on the lower netdev TX path.

Move the start/stop callbacks, so that representors are created/
destroyed when vNICs are fully initialized.

Fixes: 5de73ee4 ("nfp: general representor implementation")
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>