net: bcmgenet: add support for xmit_more

Delay the update of the TDMA producer index unless this is the last SKB
in a batch, or the queue is already stopped. Move the check for whether
the queue should be stopped before the xmit_more check to avoid locking
the transmit queue in case there was a SKB submitted which has xmit_more
set.

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