drivers: net: xgene: Backward compatibility with older firmware

This patch adds support when used with older firmware (<= 1.13.28).

- Added xgene_ring_mgr_init() to check whether ring manager is initialized
- Calling xgene_ring_mgr_init() from xgene_port_ops.reset()
- To handle errors, changed the return type of xgene_port_ops.reset()

Signed-off-by: Iyappan Subramanian <isubramanian@apm.com>
Signed-off-by: Keyur Chudgar <kchudgar@apm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>