net: txgbe: add FW version warning

Since XPCS device identifier is implemented in the firmware version
0x20010 and above, so add a warning to prompt the users to upgrade the
firmware to make sure the driver works.

Signed-off-by: Jiawen Wu <jiawenwu@trustnetic.com>
Signed-off-by: David S. Miller <davem@davemloft.net>