extmod/uasyncio: Fix syntax of generator functions.

The compiler is not picky right now, but these are actually all syntax
errors:
- await is only valid in an async function
- async functions that use yield are actually async generators (a construct
  not supported by the compiler right now)

extmod/uasyncio/lock.py

@@ -28,7 +28,8 @@ class Lock:
             # No Task waiting so unlock
             self.state = 0
 
-    async def acquire(self):
+    # async
+    def acquire(self):
         if self.state != 0:
             # Lock unavailable, put the calling Task on the waiting queue
             self.waiting.push(core.cur_task)