UBI: fix ubi free PEBs count calculation

The ubi->free_count should be updated with every insert/remove to/from
the ubi->free list.

Signed-off-by: Tanya Brokhman <tlinder@codeaurora.org>
Reviewed-by: Dolev Raviv <draviv@codeaurora.org>
Acked-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>