mtd: initialize writebufsize in the MTD object of a partition

Propagate the writebufsize to the partition's MTD object so
that UBI can set correct value for it's minimal I/O size
using the writebufsize field of MTD object of the partition.

By previous patches we added proper writebufsize field
initialization. Next patch can now change UBI to use
this field for setting the minimal I/O size.

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>