With onfi a flash is organized into one or more logical units (LUNs).
A logical unit (LUN) is the minimum unit that can independently execute
commands and report status.
Mtd does not exploit LUN, so make it see a big single flash where size is
lun_size * number_of_lun.
Without this patch MT29F8G08ADBDAH4 size is 512MiB instead of 1GiB.