drivers/nble: Fix passing uninitialized scan parameters

Initialize req to zero, fixing passing uninitialized use_whitelist,
scan_type in nble_gap_connect_req().

Change-Id: I57f957db81f6d7cd3e24e3e7393ba1f055b42330
Signed-off-by: Andrei Emeltchenko <andrei.emeltchenko@intel.com>