Bluetooth: Don't delay bt_conn connected callback

Delaying only connected callback resulted in races where security
or pairing callback could be called before bt_conn connected.

Currently only procedure that relies on read features is connection
parameters update and that should not conflict with other traffic.

Change-Id: I835535abbff21ea8d86b728f5681f90ef9fd337c
Signed-off-by: Szymon Janc <ext.szymon.janc@tieto.com>