Bluetooth: Move mgmt event sending out from hci_add_link_key()

There are two callers of hci_add_link_key(). The first one is the HCI
Link Key Notification event and the second one the mgmt code that
receives a list of link keys from user space. Previously we've had the
hci_add_link_key() function being responsible for also emitting a mgmt
signal but for the latter use case this should not happen. Because of
this a rather awkward new_key paramter has been passed to the function.

This patch moves the mgmt event sending out from the hci_add_link_key()
function, thereby making the code a bit more understandable.

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