dpaa2-eth: invoke dpaa2_eth_enable_tx_tstamp() once in code

Invoke dpaa2_eth_enable_tx_tstamp() once in code after building FD,
rather than calling it in dpaa2_eth_build_single_fd(),
dpaa2_eth_build_sg_fd_single_buf(), and dpaa2_eth_build_sg_fd().

Signed-off-by: Yangbo Lu <yangbo.lu@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>