mtd: adapt writebufsize calculation for concat

If different chips with different writebufsize are concatenated,
the writebufsize from the concat device has to be taken from
the device with the largest writebuffer. This writebufsize
is used later on in the UBI layer for the min I/O size.

Signed-off-by: Holger Brunck <holger.brunck@keymile.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>