gpiolib: fix crash when gpiochip removed

gpiolib: fix crash when gpiochip removed

Commit cb464a88e1ed ("gpio: make the gpiochip a real device") call
gpiochip_sysfs_unregister after the gpiochip is empty. It lead to the
following crash:

[  163.503994] Unable to handle kernel NULL pointer dereference at virtual address 0000007c
[...]
[  163.525394] [<ffffffc0003719a0>] gpiochip_sysfs_unregister+0x44/0xa4
[  163.525611] [<ffffffc00036f6a0>] gpiochip_remove+0x24/0x154
[  163.525861] [<ffffffbffc00f0a4>] mockup_gpio_remove+0x38/0x64 [gpio_mockup]
[  163.526101] [<ffffffc00042b4b4>] platform_drv_remove+0x24/0x64
[  163.526313] [<ffffffc000429cc8>] __device_release_driver+0x7c/0xfc
[  163.526525] [<ffffffc000429e54>] driver_detach+0xbc/0xc0
[  163.526700] [<ffffffc000429014>] bus_remove_driver+0x58/0xac
[  163.526883] [<ffffffc00042a4cc>] driver_unregister+0x2c/0x4c
[  163.527067] [<ffffffc00042b5c0>] platform_driver_unregister+0x10/0x18
[  163.527284] [<ffffffbffc00f340>] mock_device_exit+0x10/0x38 [gpio_mockup]
[  163.527593] [<ffffffc00011cefc>] SyS_delete_module+0x1b8/0x1fc
[  163.527799] [<ffffffc000085d8c>] __sys_trace_return+0x0/0x4
[  163.528049] Code: 940d74b4 f9019abf aa1303e0 940d7439 (7940fac0)
[  163.536273] ---[ end trace 3d1329be504af609 ]---

This patch fix this by changing the code back.

Signed-off-by: Bamvor Jian Zhang <bamvor.zhangjian@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>