net: axienet: Be more careful about updating tx_bd_tail

The axienet_start_xmit function was updating the tx_bd_tail variable
multiple times, with potential rollbacks on error or invalid
intermediate positions, even though this variable is also used in the
TX completion path. Use READ_ONCE where this variable is read and
WRITE_ONCE where it is written to make this update more atomic, and
move the write before the MMIO write to start the transfer, so it is
protected by that implicit write barrier.

Signed-off-by: Robert Hancock <robert.hancock@calian.com>
Signed-off-by: David S. Miller <davem@davemloft.net>