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>