Bluetooth: mgmt: Use the correct print format

According to Documentation/core-api/printk-formats.rst,
Use the correct print format. Printing an unsigned int value should use %u
instead of %d. Otherwise printk() might end up displaying negative numbers.

Signed-off-by: Kai Ye <yekai13@huawei.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>