mtd, cfi, ubi: add missing writebufsize initialization

ff94bc40 "mtd, ubi, ubifs: resync with Linux-3.14"
introduced the writebufsize field in struct mtd_info, which
is not initialized in the cfi_flash driver, which leads in
not working ubi on cfi flashes. Fix it

Signed-off-by: Heiko Schocher <hs@denx.de>
Reported-by: Andrew Ruder <andrew.ruder@elecsyscorp.com>
Acked-by: Stefan Roese <sr@denx.de>
Acked-by: Andrew Ruder <andrew.ruder@elecsyscorp.com>