Bluetooth: controller: fix failing EBQ advertising tests

Updates the chaining for advertising.
Instead of unconditionally adding a new PDU when new data
is added we now instead fill the last PDU in the chain with
the incoming data, only adding a new PDU when there is
not enough room.

This reduces the nr. of PDUs used for advertising, and also
fixes some qualification failures

Signed-off-by: Andries Kruithof <andries.kruithof@nordicsemi.no>