Wifi: Fix vts tests error handling

In current code, VTS tests checks on null pointer using EXPECT_NE
then proceed with using the variable. This causes a null pointer
exception when the test fails.
This commit replaces the EXPECT_NE with ASSERT_NE to exit the test
on failure.

Bug: 152576797
Test: atest VtsHalWifiV1_0TargetTest
Change-Id: I5e54634020216f91144a234caf2b990de5706d8c