net: ethernet: mtk_eth_soc: rely on GFP_KERNEL for dma_alloc_coherent whenever possible

Rely on GFP_KERNEL for dma descriptors mappings in mtk_tx_alloc(),
mtk_rx_alloc() and mtk_init_fq_dma() since they are run in non-irq
context.

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>