nfp: flower: base lifetime of representors on existence of lower vNIC

Create representors after lower vNIC is registered and destroy
them before it is destroyed.  Move the code out of start/stop
callbacks directly into vnic_init/clean callbacks.  Make sure
SR-IOV callbacks don't try to create representors when lower
device does not exist.

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>