Bluetooth: Fix to handle multiple conn_update_requests

The state machine for conn update requests from the peripheral,
updates the state to UPDATED, once it accepts and sends the connection
update request. However this state is never updated back to ENABLED
due to which any more connection requests get filtered.
The fix restores back the state to ENABLED once the LE connection
update complete is received.
CRs-Fixed: 532698

Change-Id: Iac3015b4aed2213876f6a1150cd4619f4d970f6b
(cherry picked from commit 7e5916e1e3daf3b8d916f88ea0fd9739f0097aa4)
(cherry picked from commit ae953dbb3a016a7e35ae5248d714cc3a16597dc1)
(cherry picked from commit 439e1d9078115065266cfe46b377e48de89526f1)