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/src/hal_sleep.c

@@ -57,6 +57,9 @@ int sleep(const uint8_t mode)
 	if (ERR_NONE != _set_sleep_mode(mode))
 		return ERR_INVALID_ARG;
 
+        // wait for the mode set to actually take, per chip doc.
+        while(_get_sleep_mode() != mode);
+
 	_go_to_sleep();
 
 	return ERR_NONE;