ravb: remove unhandle int cause

This driver does not handle the AVB-DMAC Receive FIFO Warning interrupt
now, so the interrupt should not be enabled.

Signed-off-by: Masaru Nagai <masaru.nagai.vx@renesas.com>
Signed-off-by: Yoshihiro Kaneko <ykaneko0929@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>