extmod/asyncio/event.py: Fix ThreadSafeFlag.ioctl return.

iobase_ioctl expects that an ioctl method must return an integer, and will
raise otherwise.

This was tripping up aioble on Unix, where the new hybrid modselect.c
implementation will attempt to extract a file descriptor from the pollable
via ioctl(MP_STREAM_GET_FILENO).

However, ThreadSafeFlag's ioctl only supported MP_STREAM_POLL, and returned
None otherwise.

This makes it return `-1` (to match tests/extmod/select_poll_custom.py). It
should probably be `-22` (corresponding to MP_EINVAL), but the value is
never checked, and MP_EINVAL can be a different value on different ports.

This work was funded through GitHub Sponsors.
Signed-off-by: Jim Mussared <jim.mussared@gmail.com>

extmod/asyncio/event.py

@@ -49,7 +49,7 @@ try:
         def ioctl(self, req, flags):
             if req == 3:  # MP_STREAM_POLL
                 return self.state * flags
-            return None
+            return -1  # Other requests are unsupported
 
         def set(self):
             self.state = 1