In asyncio_send() we assert that the passed data arg is no larger than the chip's FIFO (252 bytes) but then when radiohead is enabled, we prepend 4 bytes to data potentially making it larger than 252 bytes. This behavior results in data not being sent via the radio when it is equal to self.max_packet_length and radio head is enabled.

It feels like we should assert max packet length is 248 when radiohead is enabled. Even with the comment detailing how self.max_packet_length is used on async_send() I think this would also be a helpful note above all the locations where self.max_packet_length is set.

Thoughts?