Return error code properly
[Model] All
[BinType] AP
[Customer] OPEN
[Issue#] N/A
[Request] Internal
[Occurrence Version] N/A
[Problem] In disconnection staus, bt frwk return MTU size without error code
[Cause & Measure] When MTU size is invalid, return error code.
[Checking Method] Getting MTU size
[Team] IoT Hub
[Developer] Injun Yang
[Solution company] Samsung
[Change Type] Specification change
Change-Id: I23fa6a2a4bcb1f75c081a502062b7d5e7177111b
Signed-off-by: injun.yang <injun.yang@samsung.com>
Signed-off-by: DoHyun Pyun <dh79.pyun@samsung.com>