net: macb: apply sane DMA configuration

DMA configuration was heavily dependent on the HW
defaults, add function to properly set the required
fields, including the new dma_burst_length.

Signed-off-by: Ramon Fried <rfried.dev@gmail.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>
Tested-by: Anup Patel <anup.patel@wdc.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>