forcedeth: xmit lock fix

This patch fixes a potential race condition between xmit thread and xmit
completion thread. The calculation of empty tx descriptors is not
performed under the lock. This could cause it to set the stop flag while
the completion thread finishes all tx's. This will result in the tx
queue in stopped state and no one to wake it up.

Signed-off-by: Ayaz Abdulla <aabdulla@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>