macb: avoid lockup when TGO during underrun

In rare cases when an underrun occur, all macb buffers where consumed
and the netif_queue was stopped infinitely. This happens then the TGO
(transfer ongoing) bit in the TSR is set (and UND). It seems like
clening up after the underrun makes the driver and the macb hardware
end up in an inconsistent state. The result of this is that in the
following calls to macb_tx no TX buffers are released -> the
netif_queue was stopped, and never woken up again.

The solution is to disable the transmitter, if TGO is set, before
clening up after the underrun, and re-enable the transmitter when the
cleaning up is done.

Signed-off-by: Richard Röjfors <richard.rojfors@endian.se>
Signed-off-by: David S. Miller <davem@davemloft.net>