mtd: rawnand: brcmnand: Fix data access violation for STB chip

Florian reported the following kernel NULL pointer dereference issue on
a BCM7250 board:
[    2.829744] Unable to handle kernel NULL pointer dereference at virtual address 0000000c when read
[    2.838740] [0000000c] *pgd=80000000004003, *pmd=00000000
[    2.844178] Internal error: Oops: 206 [#1] SMP ARM
[    2.848990] Modules linked in:
[    2.852061] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 6.8.0-next-20240305-gd95fcdf4961d #66
[    2.860436] Hardware name: Broadcom STB (Flattened Device Tree)
[    2.866371] PC is at brcmnand_read_by_pio+0x180/0x278
[    2.871449] LR is at __wait_for_common+0x9c/0x1b0
[    2.876178] pc : [<c094b6cc>]    lr : [<c0e66310>]    psr: 60000053
[    2.882460] sp : f0811a80  ip : 00000012  fp : 00000000
[    2.887699] r10: 00000000  r9 : 00000000  r8 : c3790000
[    2.892936] r7 : 00000000  r6 : 00000000  r5 : c35db440  r4 : ffe00000
[    2.899479] r3 : f15cb814  r2 : 00000000  r1 : 00000000  r0 : 00000000

The issue only happens when dma mode is disabled or not supported on STB
chip. The pio mode transfer calls brcmnand_read_data_bus function which
dereferences ctrl->soc->read_data_bus. But the soc member in STB chip is
NULL hence triggers the access violation. The function needs to check
the soc pointer first.

Fixes: 546e4259

 ("mtd: rawnand: brcmnand: Add BCMBCA read data bus interface")

Reported-by: Florian Fainelli <florian.fainelli@broadcom.com>
Tested-by: Florian Fainelli <florian.fainelli@broadcom.com>
Signed-off-by: William Zhang <william.zhang@broadcom.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20240320222623.35604-1-william.zhang@broadcom.com