Bluetooth: Move bt_conn functions to conn.c

There's no reason to keep these in hci_core.c. Also remove the
duplicate bt_conn_update_param_le() and make bt_conn_set_param_le()
static since it's only accessed from within conn.c.

Change-Id: I1234e9d2e5d1d367732a7b66965cf10a338c07fb
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>