[NUCLEO_F030R8] 16-bit timer register update

This path fixes issue ARMmbed#816.

Current value of TIM_MST->CNT is read in interrupt context only.
This avoids master timer overflow without SlaveCounter update.

Change-Id: I8e2ec02ce7539a4c044c7e3dfe6bedc9fcdf7736

hal/targets/cmsis/TARGET_STM/TARGET_STM32F0/TARGET_NUCLEO_F030R8/hal_tick.c

@@ -43,6 +43,7 @@ void set_compare(uint16_t count);
 extern volatile uint32_t SlaveCounter;
 extern volatile uint32_t oc_int_part;
 extern volatile uint16_t oc_rem_part;
+extern volatile uint16_t cnt_val;
 
 // Used to increment the slave counter
 void timer_update_irq_handler(void)
@@ -59,7 +60,7 @@ void timer_update_irq_handler(void)
 // Used for mbed timeout (channel 1) and HAL tick (channel 2)
 void timer_oc_irq_handler(void)
 {
- uint16_t cval = TIM_MST->CNT;
+ cnt_val = TIM_MST->CNT;
  TimMasterHandle.Instance = TIM_MST;
 
  // Channel 1 for mbed timeout
@@ -71,7 +72,7 @@ void timer_oc_irq_handler(void)
  } else {
  if (oc_int_part > 0) {
  set_compare(0xFFFF);
- oc_rem_part = cval; // To finish the counter loop the next time
+ oc_rem_part = cnt_val; // To finish the counter loop the next time
  oc_int_part--;
  } else {
  us_ticker_irq_handler();