[developermode] Fix typo (usbIpAddress <-> usbInterface)

This bug prevented the IP address from being correctly passed from the
usb_moded D-Bus service to the developer mode settings UI, as
m_usbIpAddress was never set to the right value.
Merge pull request #15 from thp/usb-ip-fix

[developermode] Fix typo (usbIpAddress <-> usbInterface)