Avoid type collision by renaming callback variable

Build jobs are failing due build error "arm_hal_timer.cpp:50:5:
error: reference to 'callback' is ambiguous".

Fix the build error by renaming callback to arm_hal_callback to
avoid collision with callback defined in ./mbed-os/hal/api/Callback.h

Author: Arto Kinnunen

features/FEATURE_COMMON_PAL/nanostack-hal-mbed-cmsis-rtos/arm_hal_timer.cpp

@@ -16,7 +16,7 @@ static osThreadId timer_thread_id;
 static Timer timer;
 static Timeout timeout;
 static uint32_t due;
-static void (*callback)(void);
+static void (*arm_hal_callback)(void);
 
 static void timer_thread(const void *)
 {
@@ -25,7 +25,7 @@ static void timer_thread(const void *)
         // !!! We don't do our own enter/exit critical - we rely on callback
         // doing it (ns_timer_interrupt_handler does)
         //platform_enter_critical();
-        callback();
+        arm_hal_callback();
         //platform_exit_critical();
     }
 }
@@ -47,7 +47,7 @@ void platform_timer_disable(void)
 // Not called while running, fortunately
 void platform_timer_set_cb(void (*new_fp)(void))
 {
-    callback = new_fp;
+    arm_hal_callback = new_fp;
 }
 
 static void timer_callback(void)