cpu/nrf51822: fixed timers

- changed TIMER_0 to be a 24-bit timer -> see Nordic PAN #32
- cleaned up timer initialization code
- added check if an interrupt channel was active

cpu/nrf51822/include/hwtimer_cpu.h

@@ -9,10 +9,10 @@
 /**
  * @ingroup         cpu_nrf51822
  * @{
- * 
+ *
  * @file
  * @brief           CPU specific hwtimer configuration options
- * 
+ *
  * @author          Hauke Petersen <hauke.peterse@fu-berlin.de>
  */
 
@@ -26,7 +26,7 @@
 #define HWTIMER_MAXTIMERS   3               /**< the CPU implementation supports 3 HW timers */
 
 #define HWTIMER_SPEED       1000000         /**< the HW timer runs with 1MHz */
-#define HWTIMER_MAXTICKS    (0xFFFFFFFF)    /**< 32-bit timer */
+#define HWTIMER_MAXTICKS    (0xFFFFFF)      /**< 24-bit timer -> see PAN note */
 /** @} */