drivers: net: xgene: Fix module unload crash - hw resource cleanup

When the driver is configured as kernel module and when it gets
unloaded and reloaded, kernel crash was observed.  This patch
address the hardware resource cleanups by doing the following,

- Added mac_ops->clear() to do prefetch buffer clean up
- Fixed delete freepool buffers logic
- Reordered mac_enable and mac_disable
- Added Tx completion ring free
- Moved down delete_desc_rings after ring cleanup

Signed-off-by: Iyappan Subramanian <isubramanian@apm.com>
Tested-by: Fushen Chen <fchen@apm.com>
Tested-by: Toan Le <toanle@apm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>