net: macb: move the Tx and Rx buffer initialization into a function

This patch moves the Tx and Rx buffer initialization into its own
function. This does not modify the behaviour of the driver and will be
helpful to convert the driver to phylink.

Signed-off-by: Antoine Tenart <antoine.tenart@bootlin.com>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>