watchdog: dw_wdt: No need for a spinlock

Right now the dw_wdt uses a spinlock to protect dw_wdt_open(). The
problem is that while holding the spinlock we call:
-> dw_wdt_set_top()
-> dw_wdt_top_in_seconds()
-> clk_get_rate()
-> clk_prepare_lock()
-> mutex_lock()

Locking a mutex while holding a spinlock is not allowed and leads to
warnings like "BUG: spinlock wrong CPU on CPU#1", among other
problems.

There's no reason to use a spinlock. Only dw_wdt_open() was protected
and the test_and_set_bit() at the start of that function protects us
anyway.

Signed-off-by: Doug Anderson <[e-mail]>
Tested-by: Jisheng Zhang <[e-mail]>
Reviewed-by: Dmitry Torokhov <[e-mail]>
Reviewed-by: Guenter Roeck <[e-mail]>
Signed-off-by: Wim Van Sebroeck <[e-mail]>

drivers/watchdog/dw_wdt.c

···
 #include <linux/pm.h>
 #include <linux/platform_device.h>
 #include <linux/reboot.h>
-#include <linux/spinlock.h>
 #include <linux/timer.h>
 #include <linux/uaccess.h>
 #include <linux/watchdog.h>
···
 #define WDT_TIMEOUT		(HZ / 2)
 
 static struct {
-	spinlock_t		lock;
 	void __iomem		*regs;
 	struct clk		*clk;
 	unsigned long		in_use;
···
 	/* Make sure we don't get unloaded. */
 	__module_get(THIS_MODULE);
 
-	spin_lock(&dw_wdt.lock);
 	if (!dw_wdt_is_enabled()) {
 		/*
 		 * The watchdog is not currently enabled. Set the timeout to
···
 	}
 
 	dw_wdt_set_next_heartbeat();
-
-	spin_unlock(&dw_wdt.lock);
 
 	return nonseekable_open(inode, filp);
 }
···
 	ret = clk_prepare_enable(dw_wdt.clk);
 	if (ret)
 		return ret;
-
-	spin_lock_init(&dw_wdt.lock);
 
 	ret = misc_register(&dw_wdt_miscdev);
 	if (ret)