amd-xgbe: Use the proper register during PTP initialization

During PTP initialization, the Timestamp Control register should be
cleared and not the Tx Configuration register.  While this typo causes
the wrong register to be cleared, the default value of each register and
and the fact that the Tx Configuration register is programmed afterwards
doesn't result in a bug, hence only fixing in net-next.

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