Bluetooth: kfree tmp rather than an alias to it

While the kfree of dhkey_a is of the same address of tmp, it
probably is clearer and more human readable if tmp is kfree'd
rather than dhkey_a.

Detected by CoverityScan, CID#1448650 ("Free of address-of expression")
Signed-off-by: NColin Ian King <colin.king@canonical.com>
Signed-off-by: NMarcel Holtmann <marcel@holtmann.org>

net/bluetooth/selftest.c

@@ -164,7 +164,7 @@ static int __init test_ecdh_sample(const u8 priv_a[32], const u8 priv_b[32],
 		ret = -EINVAL;
 
 out:
-	kfree(dhkey_a);
+	kfree(tmp);
 	return ret;
 }