gianfar: Fix BD_LENGTH_MASK definition

BD_LENGTH_MASK is supposed to catch the low 16-bits of the status field, not
the low byte.  The old way, we would never be able to clean up tx packets with
sizes divisible by 256.

Signed-off-by: Andy Fleming <afleming@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>