mtd: ubi: Make in_wl_tree function public

Make the in_wl_tree function public to be used outside of wl.c.
Rename it to ubi_in_wl_tree.

Change-Id: I939a96c83ec64f845d8142cdee812f7b630e259a
Signed-off-by: Tanya Brokhman <tlinder@codeaurora.org>