sh_eth: remove unchecked interrupts for RZ/A1

When streaming a lot of data and the RZ/A1 can't keep up, some status bits
will get set that are not being checked or cleared which cause the
following messages and the Ethernet driver to stop working. This
patch fixes that issue.

irq 21: nobody cared (try booting with the "irqpoll" option)
handlers:
[<c036b71c>] sh_eth_interrupt
Disabling IRQ #21

Fixes: db893473 ("sh_eth: Add support for r7s72100")
Signed-off-by: Chris Brandt <chris.brandt@renesas.com>
Acked-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>