ubi: Replace erase_block() with sync_erase()

Since erase_block() has same logic with sync_erase(), just replace it
with sync_erase(), also rename 'sync_erase()' to 'ubi_sync_erase()'.

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