net: openthread: fix unwanted 802.15.4 radio up

This commit moves IPv6 initialization from OT init to OT start to
avoid unwantedly bringing 802.15.4 radio up.

Previously, even when OT manual start was enabled, the radio would
be receiving frames resulting in unnecessary power consumption and
causing issues for instance when the device just wants to use
Bluetooth for provisioning befor moving to Thread.

Signed-off-by: Eduardo Montoya <eduardo.montoya@nordicsemi.no>