tethering: Fix bridge module loading problem

tethering: Fix bridge module loading problem

This is back-port from upstream. Commit ID in upstream is
4fe07dfed4f720136ee142e56a9d43242c8c78dc. Fixes TIVI-166.

Connman tries to check if bridging is supported by the kernel by checking
whether "/proc/sys/net/bridge" is present. If the bridge is a kernel
module which is not already loaded, then tethering cannot be enabled.

Instead of checking for the file we invoke the "get bridge version" socket
ioctl and the kernel will handle module loading issues - if the bridge
module is not loaded, it will first load it, and then check the version.

Change-Id: I5d9e15ca2fcab384c660c8b66ab5f184067545b1