drivers/nble: Fix memory leak with double connection create

on_nble_gap_connect_evt() gets called in a case of incoming connection
but also with outcoming one. In this case connection is already
created so we should use conn_get() instead of conn_new().

Change-Id: I2ed2d0a1844f653000a12eb0f54f52533856bf0d
Signed-off-by: Andrei Emeltchenko <andrei.emeltchenko@intel.com>