CLEANUP: net_helper: fix incorrect const pointers in writev_n16()

CLEANUP: net_helper: fix incorrect const pointers in writev_n16()

It's interesting to see that output pointers p1 and p2 were declared
as const, and that thisremained unnoticed due to the explicit casts
to u8 when writing to them. The function is currently not used, but
better clean it up to avoid surprises.