pxa3xx_nand: disable nand irq in initialization

In some bootloader, IRQ is enabled. Writing nand triggers unexpected
interrupts. So disable nand irq in initialization. After nand
initialized and in working state, irq is controlled by nand driver.

Signed-off-by: Haojian Zhuang <haojian.zhuang@marvell.com>
Signed-off-by: Eric Miao <eric.y.miao@gmail.com>