Bluetooth: hci_sync: Fix not checking error on hci_cmd_sync_cancel_sync

hci_cmd_sync_cancel_sync shall check the error passed to it since it
will be propagated using req_result which is __u32 it needs to be
properly set to a positive value if it was passed as negative othertise
IS_ERR will not trigger as -(errno) would be converted to a positive
value.

Fixes: 63298d6e ("Bluetooth: hci_core: Cancel request on command timeout")
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
Reported-and-tested-by: Thorsten Leemhuis <linux@leemhuis.info>
Closes: https://lore.kernel.org/all/08275279-7462-4f4a-a0ee-8aa015f829bc@leemhuis.info/