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/hpl/pm/hpl_pm.c

@@ -63,6 +63,14 @@ int32_t _set_sleep_mode(const uint8_t mode)
 	return ERR_NONE;
 }
 
+/**
+ * \brief Get the sleep mode for the device
+ */
+int32_t _get_sleep_mode()
+{
+    return hri_pm_read_SLEEPCFG_SLEEPMODE_bf(PM);
+}
+
 /**
  * \brief Set performance level
  */