[cm3] fix doxygen for systick_get_value, add systick_get_reload

Fix the reload value in the examples:
to get a SysTick interrupt every N clock pulses: set reload to N-1
see http://infocenter.arm.com/help/topic/com.arm.doc.dui0552a/BGBEEJHC.html#BABGACGG

examples/stm32/f1/other/systick/systick.c

@@ -62,7 +62,8 @@ int main(void)
 	systick_set_clocksource(STK_CTRL_CLKSOURCE_AHB_DIV8);
 
 	/* 9000000/9000 = 1000 overflows per second - every 1ms one interrupt */
-	systick_set_reload(9000);
+	/* SysTick interrupt every N clock pulses: set reload to N-1 */
+	systick_set_reload(8999);
 
 	systick_interrupt_enable();
 

examples/stm32/f1/other/usb_hid/usbhid.c

@@ -238,7 +238,8 @@ static void hid_set_config(usbd_device *usbd_dev, u16 wValue)
 #endif
 
 	systick_set_clocksource(STK_CTRL_CLKSOURCE_AHB_DIV8);
-	systick_set_reload(100000);
+	/* SysTick interrupt every N clock pulses: set reload to N-1 */
+	systick_set_reload(99999);
 	systick_interrupt_enable();
 	systick_counter_enable();
 }