stmmac: fix Transmit Underflow error

stmmac: fix Transmit Underflow error

On some old MAC chips without COE sometime the
Transmit Underflow error is issued.

The driver aborted all the transmission process
and initialized it from scratch.
This breaks the network activity as raised by Nachiketa
on a SPEAr board.

The patch is to fix this rare underflow event.
The driver will only clear the interrupt and the Tx
DMA will go out the Suspend state as soon as the
descriptor is fetched again.
The driver will continue to bump-up the DMA FIFO threshold
that, indeed, helped somebody to prevent this kind of error
in the past as well.

Reported-by: Nachiketa Prachanda <nprachanda@ncomputing.com>
Signed-off-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>