driver core: Replace the dangerous to_root_device macro with an inline function

The original macro worked only when applied to variables named 'dev'.
While this could have been fixed by simply renaming the macro argument,
a more type-safe replacement by an inline function is preferred.

Signed-off-by: Ferenc Wagner <wferi@niif.hu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>