net: ll_temac: Fix iommu/swiotlb leak

Unmap the actual buffer length, not the amount of data received, avoiding
resource exhaustion of swiotlb (seen on x86_64 platform).

Signed-off-by: Esben Haabendal <esben@geanix.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>