drivers: net: xgene: Add separate tx completion ring

- Added wrapper functions around napi_add, napi_del, napi_enable and napi_disable
- Moved platform_get_irq function call after reading phy_mode
- Associating the new irq to tx completion for the supported ethernet interfaces

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>