stmhal: Fix bug where negative delay/udelay lead to huge delays.

A negative ms/us is now treated as a delay of 0 ms/us.  This patch also
improves the calibration of udelay.

stmhal/modpyb.c

@@ -143,22 +143,26 @@ STATIC MP_DEFINE_CONST_FUN_OBJ_0(pyb_millis_obj, pyb_millis);
 
 /// \function delay(ms)
 /// Delay for the given number of milliseconds.
-STATIC mp_obj_t pyb_delay(mp_obj_t count) {
-    HAL_Delay(mp_obj_get_int(count));
+STATIC mp_obj_t pyb_delay(mp_obj_t ms_in) {
+    machine_int_t ms = mp_obj_get_int(ms_in);
+    if (ms >= 0) {
+        HAL_Delay(ms);
+    }
     return mp_const_none;
 }
 STATIC MP_DEFINE_CONST_FUN_OBJ_1(pyb_delay_obj, pyb_delay);
 
 /// \function udelay(us)
 /// Delay for the given number of microseconds.
-STATIC mp_obj_t pyb_udelay(mp_obj_t usec) {
+STATIC mp_obj_t pyb_udelay(mp_obj_t usec_in) {
+    machine_int_t usec = mp_obj_get_int(usec_in);
+    if (usec > 0) {
         uint32_t count = 0;
-    const uint32_t utime = (168 * mp_obj_get_int(usec) / 5);
-    for (;;) {
-        if (++count > utime) {
-            return mp_const_none;
+        const uint32_t utime = (168 * usec / 4);
+        while (++count <= utime) {
         }
     }
+    return mp_const_none;
 }
 STATIC MP_DEFINE_CONST_FUN_OBJ_1(pyb_udelay_obj, pyb_udelay);