nfp: move the start/stop app callbacks back

Since representors are now created with a separate callback
start/stop app callbacks can be moved again to their original
location.  They are intended to app-specific init/clean up
over the control channel.

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>