amd-xgbe: Add ethtool show/set ring parameter support

Add ethtool support to show and set the number of the Rx and Tx ring
descriptors.  Changing the ring configuration will result in a device
restart.

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