amd-xgbe: Prepare for ethtool set-channel support

In order to support being able to dynamically set/change the number of
Rx and Tx channels, update the code to:
 - Move alloc and free of device memory into callable functions
 - Move setting of the real number of Rx and Tx channels to device startup
 - Move mapping of the RSS channels to device startup

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