hotplug: fix uninitialized variable

hotplug: fix uninitialized variable

Commit e999ab7 (hotplug: Check chown return value) introduced a new variable
"ret" which is not explicitely initialized, leading to the following compile
error:

    hotplug.c:155:18: error: 'ret' may be used uninitialized in this function [-Werror=maybe-uninitialized]
        if (!g || ret < 0)
                      ^
    cc1: all warnings being treated as errors

Explicitely initialize it to zero to avoid that problem.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>