amd-xgbe-phy: Fix build break for missing declaration

A previous patch inadvertently deleted a declaration in the
amd_xgbe_an_tx_training function causing the build to fail.

Add the declaration for 'priv' back to the function.

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