uasyncio.core: call_soon(): Use current time, not zero time.

Using static zero time may become a problem when using wrap-around time
sources, like utime.ticks_ms().

uasyncio.core/uasyncio/core.py

@@ -25,7 +25,7 @@ class EventLoop:
         # CPython asyncio incompatibility: we don't return Task object
 
     def call_soon(self, callback, *args):
-        self.call_at(0, callback, *args)
+        self.call_at(self.time(), callback, *args)
 
     def call_later(self, delay, callback, *args):
         self.call_at(self.time() + delay, callback, *args)