ibmvnic: Move queue restarting in ibmvnic_tx_complete

Restart of the subqueue should occur outside of the loop processing
any tx buffers instead of doing this in the middle of the loop.

Signed-off-by: Nathan Fontenot <nfont@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>