samd/mphalport: Simplify mp_hal_delay_ms().

Do NOT use `mp_hal_delay_us()` for short delays.  This was initially done
to make short delays precise, but it does not allow for scheduling.  Leave
using `mp_hal_delay_us()` to user code if needed.

Signed-off-by: robert-hh <robert@hammelrath.com>

docs/samd/quickref.rst

@@ -65,6 +65,8 @@ Use the :mod:`time <time>` module::
     start = time.ticks_ms() # get millisecond counter
     delta = time.ticks_diff(time.ticks_ms(), start) # compute time difference
 
+Note that :func:`time.sleep_us()` delays by busy waiting. During that time, other tasks are
+not scheduled.
 
 Clock and time
 --------------

ports/samd/mphalport.c

@@ -69,13 +69,9 @@ void mp_hal_clr_pin_mux(mp_hal_pin_obj_t pin) {
 }
 
 void mp_hal_delay_ms(mp_uint_t ms) {
-    if (ms > 10) {
-        uint32_t t0 = systick_ms;
-        while (systick_ms - t0 < ms) {
-            MICROPY_EVENT_POLL_HOOK
-        }
-    } else {
-        mp_hal_delay_us(ms * 1000);
+    uint32_t t0 = systick_ms;
+    while (systick_ms - t0 < ms) {
+        MICROPY_EVENT_POLL_HOOK
     }
 }