mtd: cfi: fix writebufsize initialization

When initializing mtd->writebufsize, we must take into account
possible flash chip interleaving. Wrong writebufsize initialization
caused UBIFS recovery issues resulting in unmountable UBIFS file
system on NOR flash partitions.

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