[WATCHDOG] fix watchdog/ixp4xx_wdt.c compilation

This patch fixes the following compile error caused by
commit 20d35f3e
([WATCHDOG 22/57] ixp4xx_wdt: unlocked_ioctl):

<--  snip  -->

...
  CC      drivers/watchdog/ixp4xx_wdt.o
ixp4xx_wdt.c:32: error: expected '=', ',', ';', 'asm' or '__attribute__' 
ixp4xx_wdt.c: In function 'wdt_enable':
ixp4xx_wdt.c:41: error: 'wdt_lock' undeclared (first use in this 
ixp4xx_wdt.c:41: error: (Each undeclared identifier is reported only 
ixp4xx_wdt.c:41: error: for each function it appears in.)
ixp4xx_wdt.c: In function 'wdt_disable':
ixp4xx_wdt.c:52: error: 'wdt_lock' undeclared (first use in this 
ixp4xx_wdt.c: In function 'ixp4xx_wdt_init':
ixp4xx_wdt.c:186: error: 'wdt_lock' undeclared (first use in this 
make[3]: *** [drivers/watchdog/ixp4xx_wdt.o] Error 1

<--  snip  -->
Reported-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>

drivers/watchdog/ixp4xx_wdt.c

@@ -29,7 +29,7 @@ static int nowayout = WATCHDOG_NOWAYOUT;
 static int heartbeat = 60;	/* (secs) Default is 1 minute */
 static unsigned long wdt_status;
 static unsigned long boot_status;
-static spin_lock_t wdt_lock;
+static DEFINE_SPINLOCK(wdt_lock);
 
 #define WDT_TICK_RATE (IXP4XX_PERIPHERAL_BUS_CLOCK * 1000000UL)