gianfar: fix signedness issue

irq_of_parse_and_map() has an unsigned return type.
Testing for a negative error value doesn't work here.
Signed-off-by: Nicolas Kaiser <nikai@nikai.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/gianfar.c

@@ -577,11 +577,10 @@ static int gfar_parse_group(struct device_node *np,
 			irq_of_parse_and_map(np, 1);
 		priv->gfargrp[priv->num_grps].interruptError =
 			irq_of_parse_and_map(np,2);
-		if (priv->gfargrp[priv->num_grps].interruptTransmit < 0 ||
-			priv->gfargrp[priv->num_grps].interruptReceive < 0 ||
-			priv->gfargrp[priv->num_grps].interruptError < 0) {
+		if (priv->gfargrp[priv->num_grps].interruptTransmit == NO_IRQ ||
+		    priv->gfargrp[priv->num_grps].interruptReceive  == NO_IRQ ||
+		    priv->gfargrp[priv->num_grps].interruptError    == NO_IRQ)
 			return -EINVAL;
-		}
 	}
 
 	priv->gfargrp[priv->num_grps].grp_id = priv->num_grps;