net: stmmac: prepare dma op mode config for multiple queues

This patch prepares DMA Operation Mode configuration for multiple queues.
The work consisted on breaking the DMA operation Mode configuration function
into RX and TX scope and adapting its mechanism in stmmac_main.

Signed-off-by: Joao Pinto <jpinto@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>