net: fec: fix spin_lock dead lock

=========================================================
[ INFO: possible irq lock inversion dependency detected ]
3.8.0-rc5+ #82 Not tainted
---------------------------------------------------------
swapper/0/0 just changed the state of lock:
 (&(&fep->hw_lock)->rlock){..-...}, at: [<8034e2f8>] fec_enet_start_xmit+0x48/0x                      2cc
but this lock took another, SOFTIRQ-unsafe lock in the past:
(prepare_lock){+.+.+.}

and interrupts could create inverse lock ordering between them.
other info that might help us debug this:
Possible interrupt unsafe locking scenario:

CPU0				CPU1
----				----
lock(prepare_lock);
				local_irq_disable()
				lock(&(&fep->hw_lock)->rlock);
				lock(prepare_lock);
<Interrupt>
lock(&(&fep->hw_lock)->rlock);

*** DEADLOCK ***
Signed-off-by: Frank Li <Frank.Li@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/freescale/fec_ptp.c

@@ -104,7 +104,7 @@ void fec_ptp_start_cyclecounter(struct net_device *ndev)
 	unsigned long flags;
 	int inc;
 
-	inc = 1000000000 / clk_get_rate(fep->clk_ptp);
+	inc = 1000000000 / fep->cycle_speed;
 
 	/* grab the ptp lock */
 	spin_lock_irqsave(&fep->tmreg_lock, flags);
@@ -363,6 +363,8 @@ void fec_ptp_init(struct net_device *ndev, struct platform_device *pdev)
 	fep->ptp_caps.settime = fec_ptp_settime;
 	fep->ptp_caps.enable = fec_ptp_enable;
 
+	fep->cycle_speed = clk_get_rate(fep->clk_ptp);
+
 	spin_lock_init(&fep->tmreg_lock);
 
 	fec_ptp_start_cyclecounter(ndev);