amd-xgbe-phy: Updates to KR training initiation

As part of changing rates to KR mode, KR training is initiated. If
the KR training is restarted it is possible to enter an invalid logic
state. This can be avoided by asserting a training reset bit before
initiating the KR training and then clearing the training reset bit.

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