only apply mask to _led_buffer if < 256

This works on CPython.  I _think_ it will work on CircuitPython, since
the old behavior is effectively a no-op there.  On CPython it was
throwing a `ValueError`.

I want to be clear that I don't understand this code, so I'm definitely
not confident that this change is the Correct Thing.

Author: Brennen Bearnes

adafruit_trellis.py

@@ -91,7 +91,8 @@ def __setitem__(self, x, value):
         led = ledLUT[x % 16] >> 4
         mask = 1 << (ledLUT[x % 16] & 0x0f)
         if value:
-            self._parent._led_buffer[x // 16][(led * 2) + 1] |= mask
+            if mask < 256:
+                self._parent._led_buffer[x // 16][(led * 2) + 1] |= mask
             self._parent._led_buffer[x // 16][(led * 2) + 2] |= mask >> 8
         elif not value:
             self._parent._led_buffer[x // 16][(led * 2) + 1] &= ~mask