BUG - BT USS Service write errors when MTU size is to small

[Tjaž Vračko]

Describe the bug

If a packet that would be notified as a response to write operation (for example 0x04 (list) is larger than the MTU size, prv_send_notification will return an error, and thus BT_ATT_ERR_UNLIKELY will be returned to the central device.

Firmware/Software Version
v1.3.4 of irnas-usersettings-lib

To Reproduce
Steps to reproduce the behavior:

1. Connect to device from nrf connect app
2. Make sure MTU size is low (23).
3. Subscribe to USS notifications
4. Send 0x04 command
5. If a setting has a longer name or value, it will exeede 23 bytes when encoded and will trigger the bug.

How to fix

Two paths:

1. Return a better error message to indicate MTU size.
2. if packet is to large, split it and notify it in chunks.