net: ethernet: aquantia: Fixed incorrect buff->len calculation.

rxd_wb->pkt_len is the total length of the packet.
If we received a large packet (with length > AQ_CFG_RX_FRAME_MAX) then we
will get multiple buffers. We need to fix the length of the last buffer.

Signed-off-by: Pavel Belous <pavel.belous@aquantia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>