designs: mctp-kernel: Don't reference connect() semantics in sendto()

In correcting the connect() semantics, we will significantly change the
above section, so make sendto stand alone.

Change-Id: I65e747ef68df56ceb2b5e6aa98e3569e5dfe5afe
Signed-off-by: Jeremy Kerr <jk@codeconstruct.com.au>