mtd: gpmi: fix the NULL pointer

mtd: gpmi: fix the NULL pointer

The imx23 board will check the fingerprint, so it will call the
mx23_check_transcription_stamp. This function will use @chip->buffers->databuf
as its buffer which is allocated in the nand_scan_tail().

Unfortunately, the mx23_check_transcription_stamp is called before the
nand_scan_tail(). So we will meet a NULL pointer bug:

--------------------------------------------------------------------
[    1.150000] NAND device: Manufacturer ID: 0xec, Chip ID: 0xd7 (Samsung NAND 4GiB 3,3V 8-bit), 4096MiB, page size: 4096, OOB size: 8
[    1.160000] Unable to handle kernel NULL pointer dereference at virtual address 000005d0
[    1.170000] pgd = c0004000
[    1.170000] [000005d0] *pgd=00000000
[    1.180000] Internal error: Oops: 5 [#1] ARM
[    1.180000] Modules linked in:
[    1.180000] CPU: 0 PID: 1 Comm: swapper Not tainted 3.12.0 #89
[    1.180000] task: c7440000 ti: c743a000 task.ti: c743a000
[    1.180000] PC is at memcmp+0x10/0x54
[    1.180000] LR is at gpmi_nand_probe+0x42c/0x894
[    1.180000] pc : [<c025fcb0>]    lr : [<c02f6a68>]    psr: 20000053
[    1.180000] sp : c743be2c  ip : 600000d3  fp : ffffffff
[    1.180000] r10: 000005d0  r9 : c02f5f08  r8 : 00000000
[    1.180000] r7 : c75858a8  r6 : c75858a8  r5 : c7585b18  r4 : c7585800
[    1.180000] r3 : 000005d0  r2 : 00000004  r1 : c05c33e4  r0 : 000005d0
[    1.180000] Flags: nzCv  IRQs on  FIQs off  Mode SVC_32  ISA ARM  Segment kernel
[    1.180000] Control: 0005317f  Table: 40004000  DAC: 00000017
[    1.180000] Process swapper (pid: 1, stack limit = 0xc743a1c0)
--------------------------------------------------------------------

This patch rearrange the init procedure:
   Set the NAND_SKIP_BBTSCAN to skip the nand scan firstly, and after we
   set the proper settings, we will call the chip->scan_bbt() manually.

Cc: stable@vger.kernel.org # 3.12
Signed-off-by: Huang Shijie <b32955@freescale.com>
Reported-by: Fabio Estevam <festevam@gmail.com>
Tested-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>