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: NHannes Eder <hannes@hanneseder.net>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ns83820.c

@@ -409,7 +409,7 @@ static int lnksts = 0;		/* CFG_LNKSTS bit polarity */
 struct rx_info {
 	spinlock_t	lock;
 	int		up;
-	long		idle;
+	unsigned long	idle;
 
 	struct sk_buff	*skbs[NR_RX_DESC];