Bluetooth: controller: Fix LLCP enc. execution ctx

rp_enc_state_wait_ltk_reply() is executed in thread context, so
it is not allowed to move the encryption state machine forward.

Defer LTK reply handling to next prepare event by introducing a continue
state.

Update the unit test to reflect this, and remove the TODO that actually
said there was an issue in the first place.

Signed-off-by: Thomas Ebert Hansen <thoh@oticon.com>