ubi: Fix ubi_io_write_vid_hdr so it can be used before wl init

The function ubi_io_write_vid_hdr is called in ubi attach before
the WL init if atleast one of the vtble is missing in the partition
that is beign attached.

Fix the ubi_io_write_vid_hdr function to not reference any WL code
until the WL subsystem is initialised.

Change-Id: Ia22f08cec41f0fa86d537b990e56f2852182a532
Signed-off-by: Nikhilesh Reddy <reddyn@codeaurora.org>