Bluetooth: Mesh: Fix calling send start for multi-segmented messages

Using the start callback, especially with multi-segment messages, may
not be super useful for applications, but we should support if if they
do provide it. One application could e.g. be to calculate the duration
it takes for a multi-segment message to be completely received by the
remote end.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>