ubi: fastmap: Remove unneeded break condition while filling pools

Change pool filling stop condition. Commit d09e9a2b ("ubi:
fastmap: Fix high cpu usage of ubi_bgt by making sure wl_pool
not empty") reserves fastmap data PEBs after filling 1 PEB in
wl_pool. Now wait_free_pebs_for_pool() makes enough free PEBs
before filling pool, there will still be at least 1 PEB in pool
and 1 PEB in wl_pool after doing ubi_refill_pools().

Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>