net: lan966x: Introduce helper functions

Introduce lan966x_fdma_tx_setup_dcb and lan966x_fdma_tx_start functions
and use of them inside lan966x_fdma_xmit. There is no functional change
in here.
They are introduced to be used when XDP_TX/REDIRECT actions are
introduced.

Signed-off-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>