Bluetooth: Fix compiler warning with selftest duration calculation

CC      net/bluetooth/selftest.o
net/bluetooth/selftest.c: In function ‘bt_selftest_init’:
net/bluetooth/selftest.c:246:3: warning: ‘duration’ may be used uninitialized in this function [-Wmaybe-uninitialized]
   snprintf(test_ecdh_buffer, sizeof(test_ecdh_buffer),
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
     "PASS (%llu usecs)\n", duration);
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
net/bluetooth/selftest.c:203:21: note: ‘duration’ was declared here
  unsigned long long duration;
                     ^~~~~~~~

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

net/bluetooth/selftest.c

@@ -200,7 +200,7 @@ static int __init test_ecdh(void)
 {
 	struct crypto_kpp *tfm;
 	ktime_t calltime, delta, rettime;
-	unsigned long long duration;
+	unsigned long long duration = 0;
 	int err;
 
 	calltime = ktime_get();