mtd: nand: brcmnand: Disable prefetch by default

Brcm nand controller prefetch feature needs to be disabled
by default. Enabling affects performance on random reads as
well as dma reads.

Signed-off-by: Kamal Dasu <kdasu.kdev@gmail.com>
Fixes: 27c5b17c

 ("mtd: nand: add NAND driver "library" for Broadcom STB NAND controller")
Cc: <stable@vger.kernel.org>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>