drivers/nble: Fix NULL pointer dereference

In on_nble_gattc_discover_rsp conn->gatt_private is set to NULL prior
to calling this parser functions. This fixes this issue by passing
discover params pointer to these functions.

Change-Id: I6669cb0d16dc65d532e17f96ceb9cd94dbe6ce08
Signed-off-by: Mariusz Skamra <mariusz.skamra@tieto.com>