kernel: fix *-gpio-custom module unloading

kernel: fix *-gpio-custom module unloading

Unloading and reloading the modules fails, as platform_device_put() does not
release resources fully.

root@OpenWrt:/# insmod i2c-gpio-custom bus0=0,18,0,5
[  196.860620] Custom GPIO-based I2C driver version 0.1.1
[  196.871162] ------------[ cut here ]------------
[  196.880517] WARNING: CPU: 0 PID: 1365 at fs/sysfs/dir.c:31 0x80112158
[  196.893431] sysfs: cannot create duplicate filename '/devices/platform/i2c-gpio.0'
...
[  197.513200] kobject_add_internal failed for i2c-gpio.0 with -EEXIST, don't try to register things with the same name in the same directory.

This patch fixes it by replacing platform_device_put() to
platform_device_unregister().

Fixes: da7740853715 ("i2c-gpio-custom: minor bugfix")
Fixes: 3bc81edc70e8 ("package: fix w1-gpio-custom package (closes #6770)")

Signed-off-by: Sungbo Eo <mans0n@gorani.run>