Bluetooth: Remove unnecessary early initialization of variable

We do nothing else with the auth variable in smp_cmd_pairing_rsp()
besides passing it to tk_request() which in turn only cares about
whether one of the sides had the MITM bit set. It is therefore
unnecessary to assign a value to it until just before calling
tk_request(), and this value can simply be the bit-wise or of the local
and remote requirements.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>