nfp: add nfp_app cleanup callback and make flower use it

Add a cleanup callback for undoing what app init callback did.
Make flower allocate its private structure on init and free
it from the new callback.

While at it remember to set the app pointer to NULL on the
error path to avoid any races while probe path unwinds.

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>