dwc_eth_qos: use GFP_KERNEL in dma_alloc_coherent()

Since we are in non-atomic context here we can pass GFP_KERNEL to
dma_alloc_coherent(). This enables use of the CMA.

Signed-off-by: Rabin Vincent <rabinv@axis.com>
Signed-off-by: Lars Persson <larper@axis.com>
Signed-off-by: David S. Miller <davem@davemloft.net>