ravb: Fix typo AVB->DMAC

Fix the typo AVB->DMAC in comment, as the code following the comment
is for DMAC on Gigabit Ethernet IP.

Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Suggested-by: Sergey Shtylyov <s.shtylyov@omp.ru>
Reviewed-by: Sergey Shtylyov <s.shtylyov@omp.ru>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>