[UBI] drivers/mtd/ubi/wl.c: fix uninitialized var warning

drivers/mtd/ubi/wl.c:746: warning: 'pe' may be used uninitialized in this function

Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>