gpio-fsi driver does not unbind cleanly

[shenki]

root@palm4-bmc:/sys/devices/platform/gpio-fsi/driver# echo gpio-fsi >unbind 
[  229.011519] ------------[ cut here ]------------
[  229.014946] WARNING: CPU: 0 PID: 890 at drivers/base/core.c:818 device_release+0x94/0xa4
[  229.020844] Device 'fsi0' does not have a release() function, it is broken and must be fixed.
[  229.027063] CPU: 0 PID: 890 Comm: sh Not tainted 4.13.16-01399-gd9f9f917-dirty #5
[  229.032646] Hardware name: Generic DT based system
[  229.036209] [<80109194>] (unwind_backtrace) from [<8010647c>] (show_stack+0x20/0x24)
[  229.041899] [<8010647c>] (show_stack) from [<80582d90>] (dump_stack+0x20/0x28)
[  229.047197] [<80582d90>] (dump_stack) from [<80113588>] (__warn+0xe4/0x10c)
[  229.052291] [<80113588>] (__warn) from [<80113604>] (warn_slowpath_fmt+0x54/0x74)
[  229.057766] [<80113604>] (warn_slowpath_fmt) from [<80374690>] (device_release+0x94/0xa4)
[  229.063899] [<80374690>] (device_release) from [<80587594>] (kobject_put+0xd0/0x1ec)
[  229.069585] [<80587594>] (kobject_put) from [<80377d38>] (device_unregister+0x34/0x6c)
[  229.075398] [<80377d38>] (device_unregister) from [<804522ec>] (fsi_master_unregister+0x4c/0x58)
[  229.081828] [<804522ec>] (fsi_master_unregister) from [<8045323c>] (fsi_master_gpio_remove+0x78/0x80)
[  229.088564] [<8045323c>] (fsi_master_gpio_remove) from [<8037c2bc>] (platform_drv_remove+0x34/0x4c)
[  229.095184] [<8037c2bc>] (platform_drv_remove) from [<8037aca4>] (device_release_driver_internal+0x144/0x1e0)
[  229.102549] [<8037aca4>] (device_release_driver_internal) from [<8037ad60>] (device_release_driver+0x20/0x24)
[  229.109809] [<8037ad60>] (device_release_driver) from [<803788c4>] (unbind_store+0x84/0x108)
[  229.115985] [<803788c4>] (unbind_store) from [<80378154>] (drv_attr_store+0x30/0x3c)
[  229.121648] [<80378154>] (drv_attr_store) from [<8027a5bc>] (sysfs_kf_write+0x50/0x54)
[  229.127451] [<8027a5bc>] (sysfs_kf_write) from [<80279b9c>] (kernfs_fop_write+0xf4/0x1f4)
[  229.133443] [<80279b9c>] (kernfs_fop_write) from [<8020fdb0>] (__vfs_write+0x48/0x158)
[  229.139231] [<8020fdb0>] (__vfs_write) from [<8021007c>] (vfs_write+0xb0/0x178)
[  229.144574] [<8021007c>] (vfs_write) from [<80210258>] (SyS_write+0x58/0xbc)
[  229.149863] [<80210258>] (SyS_write) from [<80102640>] (ret_fast_syscall+0x0/0x38)
[  229.155397] ---[ end trace de43cf49c084c715 ]---