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: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>