UBI: Set free_count to zero before walking through erase list

Set free_count to zero before walking through ai->erase list
in wl_init().

Found in U-Boot as U-Boot has no workqueue/threads, it immediately
calls erase_worker(), which increase for each erased block
free_count. Without this patch, free_count gets after
this initialized to zero in wl_init(), so the free_count
variable always has the maybe wrong value 0 in U-Boot.

Signed-off-by: Heiko Schocher <hs@denx.de>
Reviewed-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Richard Weinberger <richard@nod.at>