nfp: flower: fix error path during representor creation

Don't store repr pointer to reprs array until the representor is
successfully created. This avoids message about "representor
destruction" even when it was never created. Also it cleans-up the flow.
Also, check return value after port alloc.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>