ks8851: Fix interpretation of rxlen field.

According to the Datasheet (page 52):
15-12 Reserved
11-0 RXBC Receive Byte Count
This field indicates the present received frame byte size.

The code has a bug:
                 rxh = ks8851_rdreg32(ks, KS_RXFHSR);
                 rxstat = rxh & 0xffff;
                 rxlen = rxh >> 16; // BUG!!! 0xFFF mask should be applied

Signed-off-by: Max Nekludov <Max.Nekludov@us.elster.com>
Signed-off-by: David S. Miller <davem@davemloft.net>