net: bcmgenet: use ethtool_op_get_ts_info()

This change enables the use of SW timestamping on the Raspberry Pi 4.

bcmgenet's transmit function bcmgenet_xmit() implements software
timestamping. However the SOF_TIMESTAMPING_TX_SOFTWARE capability was
missing and only SOF_TIMESTAMPING_RX_SOFTWARE was announced. By using
ethtool_ops bcmgenet_ethtool_ops() as get_ts_info(), the
SOF_TIMESTAMPING_TX_SOFTWARE capability is announced.

Similar to commit a8f5cb9e ("smsc95xx: use ethtool_op_get_ts_info()")

Signed-off-by: Ryan M. Collins <rmc032@bucknell.edu>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Doug Berger <opendmb@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>