drivers/net/ns83820.c: fix sparse warnings: fix signedness

Fix this sparse warnings:
  drivers/net/ns83820.c:479:36: warning: incorrect type in argument 2 (different signedness)
  drivers/net/ns83820.c:479:36: warning: incorrect type in argument 2 (different signedness)
  drivers/net/ns83820.c:479:36: warning: incorrect type in argument 2 (different signedness)
  drivers/net/ns83820.c:479:36: warning: incorrect type in argument 2 (different signedness)

Signed-off-by: Hannes Eder <hannes@hanneseder.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/ns83820.c b/drivers/net/ns83820.c
index e80294d..9266502 100644
--- a/drivers/net/ns83820.c
+++ b/drivers/net/ns83820.c
@@ -409,7 +409,7 @@
 struct rx_info {
 	spinlock_t	lock;
 	int		up;
-	long		idle;
+	unsigned long	idle;
 
 	struct sk_buff	*skbs[NR_RX_DESC];