Get Interface:Up returns true after "set property Interface:Up false"

[jorhol]

Describe the bug
After setting property Interface:Up to false, NCP role is changed to "Disabled" and NCP state is changed to "unknown", but when reading the property back it returns 'true'.

To Reproduce

1. Information to reproduce the behavior, including:
2. Use wpantund built using the latest commit (SHA: cf3d099)
3. Set Interface:Up to false
4. Get Interface:Up property, current output is equal to Interface:Up = true

Expected behavior
Resulting Interface:Up should be equal to false to reflect the NCP state.

Console/log output
$ sudo wpanctl setprop Interface:Up false
$ sudo wpanctl getprop Interface:Up
Interface:Up = true