Bluetooth: Set default min/max connection interval for LE slaves

For all incoming LE connections, the minimum and maximum connection
interval is a value that should be copied from the controller default
values. This allows to properly check if the resulting connection
interval of a newly established connection is in the range we are
expecting.

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