amd-xgbe: Move ring allocation to device open

Move the channel and ring tracking structures allocation to device
open.  This will allow for future support to vary the number of Tx/Rx
queues without unloading the module.

Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: David S. Miller <davem@davemloft.net>