data_smart: fix resetting of reference on variablehistory

data_smart: fix resetting of reference on variablehistory

There is no "datasmart" member, only dataroot. This dates back to the
original implementation of variable history support - it's surprising we
haven't noticed the issue until now, but I guess it's rare to change a
copy of a datastore in a manner which using the old reference would
cause an issue.

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>