gpio: Fix crash due to registration race

gpiochip_add_data_with_key() adds the gpiochip to the gpio_devices list
before of_gpiochip_add() is called, but it's only the latter which sets
the ->of_xlate function pointer.  gpiochip_find() can be called by
someone else between these two actions, and it can find the chip and
call of_gpiochip_match_node_and_xlate() which leads to the following
crash due to a NULL ->of_xlate().

 Unhandled prefetch abort: page domain fault (0x01b) at 0x00000000
 Modules linked in: leds_gpio(+) gpio_generic(+)
 CPU: 0 PID: 830 Comm: insmod Not tainted 4.18.0+ #43
 Hardware name: ARM-Versatile Express
 PC is at   (null)
 LR is at of_gpiochip_match_node_and_xlate+0x2c/0x38
 Process insmod (pid: 830, stack limit = 0x(ptrval))
  (of_gpiochip_match_node_and_xlate) from  (gpiochip_find+0x48/0x84)
  (gpiochip_find) from  (of_get_named_gpiod_flags+0xa8/0x238)
  (of_get_named_gpiod_flags) from  (gpiod_get_from_of_node+0x2c/0xc8)
  (gpiod_get_from_of_node) from  (devm_fwnode_get_index_gpiod_from_child+0xb8/0x144)
  (devm_fwnode_get_index_gpiod_from_child) from  (gpio_led_probe+0x208/0x3c4 [leds_gpio])
  (gpio_led_probe [leds_gpio]) from  (platform_drv_probe+0x48/0x9c)
  (platform_drv_probe) from  (really_probe+0x1d0/0x3d4)
  (really_probe) from  (driver_probe_device+0x78/0x1c0)
  (driver_probe_device) from  (__driver_attach+0x120/0x13c)
  (__driver_attach) from  (bus_for_each_dev+0x68/0xb4)
  (bus_for_each_dev) from  (bus_add_driver+0x1a8/0x268)
  (bus_add_driver) from  (driver_register+0x78/0x10c)
  (driver_register) from  (do_one_initcall+0x54/0x1fc)
  (do_one_initcall) from  (do_init_module+0x64/0x1f4)
  (do_init_module) from  (load_module+0x2198/0x26ac)
  (load_module) from  (sys_finit_module+0xe0/0x110)
  (sys_finit_module) from  (ret_fast_syscall+0x0/0x54)

One way to fix this would be to rework the hairy registration sequence
in gpiochip_add_data_with_key(), but since I'd probably introduce a
couple of new bugs if I attempted that, simply add a check for a
non-NULL of_xlate function pointer in
of_gpiochip_match_node_and_xlate().  This works since the driver looking
for the gpio will simply fail to find the gpio and defer its probe and
be reprobed when the driver which is registering the gpiochip has fully
completed its probe.

Signed-off-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>