Bluetooth: Read LE remote features during connection establishment

When establishing a Bluetooth LE connection, read the remote used
features mask to determine which features are supported. This was
not really needed with Bluetooth 4.0, but since Bluetooth 4.1 and
also 4.2 have introduced new optional features, this becomes more
important.

This works the same as with BR/EDR where the connection enters the
BT_CONFIG stage and hci_connect_cfm call is delayed until the remote
features have been retrieved. Only after successfully receiving the
remote features, the connection enters the BT_CONNECTED state.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>