nrf/boards/microbit/modules: Initialize variable in microbit_sleep.

When compiling for microbit with LTO=0, a compiler error occurs due to
'ms' variable in the microbit_sleep function has not been initialized.

This patch initialize the variable to 0.

ports/nrf/boards/microbit/modules/modmicrobit.c

@@ -43,7 +43,7 @@ STATIC mp_obj_t microbit_reset_(void) {
 MP_DEFINE_CONST_FUN_OBJ_0(microbit_reset_obj, microbit_reset_);
 
 STATIC mp_obj_t microbit_sleep(mp_obj_t ms_in) {
-    mp_int_t ms;
+    mp_int_t ms = 0;
     if (mp_obj_is_integer(ms_in)) {
         ms = mp_obj_get_int(ms_in);
 #if MICROPY_FLOAT_IMPL == MICROPY_FLOAT_IMPL_FLOAT