net: ll_temac: Remove unused tx_bd_next struct field

The tx_bd_next field was included in the initial commit,
commit 92744989 ("net: add Xilinx ll_temac device driver"),
but has never had any real use.

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