net: bcmgenet: cleanup for dmadesc_set()

dmadesc_set() is used for setting the Tx buffer DMA address, length,
and status bits on a Tx ring descriptor when a frame is being Tx'ed.

Always set the Tx buffer DMA address first, before updating the length
and status bits, i.e. giving the Tx descriptor to the hardware.

The reason this is a cleanup rather than a fix is that the hardware
won't transmit anything from a Tx ring until the TDMA producer index
has been incremented. As long as the dmadesc_set() writes complete
before the TDMA producer index write, life is good.

Signed-off-by: Petri Gynther <pgynther@google.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>