net: gianfar: Avoid 16 bytes of memset

The memset on CAMx is wrong, as it actually unmasks all carry irq's,
which we clearly are not interested in.

The memset on CARx registers is just pointless, as they are W1C.

So let's just stop the memset before CAR1.

Signed-off-by: Esben Haabendal <esben@geanix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>