isotp_recv and the called pull_frags functions were violating the
net_buf API by interacting directly with net_buf fragments pulled from
a k_fifo.
This commit reworks the isotp_recv function. The currently processed
net_buf is stored in an additional context variable so that reading from
it can be continued in the next call to isotp_recv if not all fragments
could be fit into the provided uint8_t *data buffer.
Fixes#40070
Signed-off-by: Martin Jäger <martin@libre.solar>
27fc74feff