This commit fixes the send function of sx126x. The loop that reads the
iolist was not considering the offset. Therefore each iolist snippet was
being written into the first position.
The loop was also setting the payload length to the size of the iolist
snippet. Then the payload was also wrong.
With this commit an iolist is copied sequentially into the framebuffer
and the payload length is set to `iolist_size`
