net: altera_tse: fix msgdma_tx_completion on non-zero fill_level case

If fill_level was not zero and status was not BUSY,
result of "tx_prod - tx_cons - inuse" might be zero.
Subtracting 1 unconditionally results invalid negative return value
on this case.
Make sure not to return an negative value.

Signed-off-by: Tomonori Sakita <tomonori.sakita@sord.co.jp>
Signed-off-by: Atsushi Nemoto <atsushi.nemoto@sord.co.jp>
Reviewed-by: Dalon L Westergreen <dalon.westergreen@linux.intel.com>
Acked-by: Thor Thayer <thor.thayer@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Author: t-saki

drivers/net/ethernet/altera/altera_msgdma.c

@@ -145,7 +145,8 @@ u32 msgdma_tx_completions(struct altera_tse_private *priv)
 			& 0xffff;
 
 	if (inuse) { /* Tx FIFO is not empty */
-		ready = priv->tx_prod - priv->tx_cons - inuse - 1;
+		ready = max_t(int,
+			      priv->tx_prod - priv->tx_cons - inuse - 1, 0);
 	} else {
 		/* Check for buffered last packet */
 		status = csrrd32(priv->tx_dma_csr, msgdma_csroffs(status));