make the HAL sleep function obey the chip documentation

the sleep mode doesn't get set immediately, and needs to be waited upon.

watch-library/hardware/hal/include/hpl_sleep.h

@@ -70,6 +70,16 @@ extern "C" {
  */
 int32_t _set_sleep_mode(const uint8_t mode);
 
+/**
+ * \brief Get the sleep mode for the device
+ *
+ * This function gets the sleep mode for the device.
+ *
+ * \return the current value of the sleep mode configuration bits
+ */
+int32_t _get_sleep_mode(void);
+
+
 /**
  * \brief Reset MCU
  */