r8169: rename RTL8125 to RTL8125A

Realtek added new members to the RTL8125 chip family, therefore rename
RTL8125 to RTL8125a. Then we use the same chip naming as in the r8125
vendor driver.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>