Bluetooth: controller: Fix deferred Conn Update offset population

Fix deferred Connection Update offset population by
introduction of explicit states waiting for the offset
calculation to complete in the ULL_LOW context.

Fixes #29636.

The problem was, in an encrypted connection the enqueued PDU
to be transmitted is encrypt in the prepare callback by the
hardware and swapped to a different buffer for transmission;
the deferred offset population did not reflect in the
transmitted PDU as it was filled in the cleartext buffer
while encryption completed into the encrypted buffer.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>