Bluetooth: use the correct print format for L2CAP debug statements

Use the correct print format. Printing an unsigned int value should use %u
instead of %d. For details, please read document:
Documentation/core-api/printk-formats.rst

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