mtd: Unconditionally update ->fail_addr and ->addr in part_erase()

->fail_addr and ->addr can be updated no matter the result of
parent->_erase(), we just need to remove the code doing the same thing
in mtd_erase_callback() to avoid adjusting those fields twice.

Note that this can be done because all MTD users have been converted to
not pass an erase_info->callback() and are thus only taking the
->addr_fail and ->addr fields into account after part_erase() has
returned.

While we're at it, get rid of the erase_info->mtd field which was only
needed to let mtd_erase_callback() get the partition device back.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Richard Weinberger <richard@nod.at>

drivers/mtd/ftl.c

@@ -342,7 +342,6 @@ static int erase_xfer(partition_t *part,
     if (!erase)
             return -ENOMEM;
 
-    erase->mtd = part->mbd.mtd;
     erase->addr = xfer->Offset;
     erase->len = 1 << part->header.EraseUnitSize;