usb_stream_helper requires the chunk buffer size to be a power of 2

[george-norton]

The asserts in the code indicate that that chunk_size must be a multiple of the usb packet size (64 bytes):

pico-extras/src/rp2_common/usb_device/usb_stream_helper.c

Line 120 in e5b9cb4

assert(!(chunk_size & 63u)); // buffer should be a multiple of USB packet buffer size

But in reality things break if the chunk_size is not also a power of 2 as the offset computation breaks down here:

pico-extras/src/rp2_common/usb_device/usb_stream_helper.c

Line 12 in e5b9cb4

return transfer->offset & (transfer->chunk_size - 1);

For example, if the chunk_size is 192 bytes the offset will not be updated after the first packet is sent, and so the first packet gets sent twice.