mtd, 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().

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.

Detected this behaviour on the dxr2 board, where the
UBI fastmap gets not written when attaching/dettaching
on an empty NAND. It drops instead the error message:

could not find any anchor PEB

With this patch, fastmap gets written on dettach.

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