kernel: 4.19: fix crash when setting up spi-gpio

kernel: 4.19: fix crash when setting up spi-gpio

If an spi-gpio was specified with num-chipselects = <0> in dts, kernel
will crash:

 Unable to handle kernel paging request at virtual address 32697073
 pgd = (ptrval)
 [32697073] *pgd=00000000
 Internal error: Oops: 5 [# 1] SMP ARM
 Modules linked in:
 CPU: 2 PID: 1 Comm: swapper/0 Not tainted 4.19.72 #0
 Hardware name: Generic DT based system
 PC is at validate_desc+0x28/0x80
 LR is at gpiod_direction_output+0x14/0x128
 ...
 [<c0544db4>] (validate_desc) from [<c0545228>] (gpiod_direction_output+0x14/0x128)
 [<c0545228>] (gpiod_direction_output) from [<c05fa714>] (spi_gpio_setup+0x58/0x64)
 [<c05fa714>] (spi_gpio_setup) from [<c05f7258>] (spi_setup+0x12c/0x148)
 [<c05f7258>] (spi_setup) from [<c05f7330>] (spi_add_device+0xbc/0x12c)
 [<c05f7330>] (spi_add_device) from [<c05f7f74>] (spi_register_controller+0x838/0x924)
 [<c05f7f74>] (spi_register_controller) from [<c05fa494>] (spi_bitbang_start+0x108/0x120)
 [<c05fa494>] (spi_bitbang_start) from [<c05faa34>] (spi_gpio_probe+0x314/0x338)
 [<c05faa34>] (spi_gpio_probe) from [<c05a844c>] (platform_drv_probe+0x34/0x70)

The cause is spi_gpio_setup() did not check if the spi-gpio has
chipselect pins before setting their direction and results in derefing
an invalid pointer.

The bug is spotted in kernel 4.19.72 and does not occur in 4.14.

There is a similar fix upstream in kernel 5.2 in commit 249e2632dcd0
("spi: gpio: Don't request CS GPIO in DT use-case").

Ref: https://patchwork.kernel.org/patch/11150619/
Signed-off-by: DENG Qingfang <dengqf6@mail2.sysu.edu.cn>
[use upstream patch, moved from hack to pending dir, commit facelift]
Signed-off-by: Petr Štetiar <ynezz@true.cz>