net: phy: nxp-c45-tja11xx: implement mdo_insert_tx_tag

Implement mdo_insert_tx_tag to insert the TLV header in the ethernet
frame.

Signed-off-by: Radu Pirea (NXP OSS) <radu-nicolae.pirea@oss.nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>