drivers: net: xgene: Fix redundant prefetch buffer cleanup

Prefetch buffer cleanup code was called twice, causing EDAC to
report errors during reboot.

[ 1130.972475] xgene-edac 78800000.edac: IOB bridge agent (BA) transaction
error
[ 1130.979584] xgene-edac 78800000.edac: IOB BA write response error
[ 1130.985648] xgene-edac 78800000.edac: IOB BA write access at 0x00.00000000
()
[ 1130.993612] xgene-edac 78800000.edac: IOB BA requestor ID 0x00002400
[ 1131.000242] xgene-edac 78800000.edac: IOB bridge agent (BA) transaction
error
...

This patch fixes the errors by,

- removing the redundant prefetch buffer cleanup from port_ops->shutdown()
- moving port_ops->shutdown() after delete_rings()

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