drivers: rk_nand: fix ota update loader fail issue

BUG:
[   98.697677] Unable to handle kernel paging request at virtual address 0000f9ff
[   98.698359] pgd = ffffff800940e000
[   98.698799] [0000f9ff] *pgd=000000003effe003, *pud=000000003effe003, *pmd=0000000000000000
[   98.699604] Internal error: Oops: 96000005 [#1] PREEMPT SMP
[   98.700116] Modules linked in:
[   98.700430] CPU: 1 PID: 152 Comm: rknand Tainted: G        W       4.4.167 torvalds#285
[   98.701085] Hardware name: Rockchip rk3326 S863 7 avb board (DT)
[   98.701630] task: ffffffc03ca71b00 task.stack: ffffffc007e40000
[   98.702192] PC is at write_loader_lba+0x188/0x1e8
[   98.702627] LR is at FtlWrite+0x40/0x70
[   98.702991] pc : [<ffffff8008881770>] lr : [<ffffff8008881810>] pstate: 20400145
[   98.703656] sp : ffffffc007e43c10
[   98.703969] x29: ffffffc007e43c10 x28: 0000000000003000
[   98.704480] x27: ffffffc007c8e280 x26: ffffffbdc0000000
[   98.704990] x25: 0000000000000000 x24: 0000000000000000

Change-Id: I9ddca6e8122f90ccc8103829d7d2b8ef89327ee7
Signed-off-by: Yifeng Zhao <[email protected]>

Author: zhao0116