Add missing platform_set_drvdata() in gpio_wdt_probe(), otherwise
calling platform_get_drvdata() in remove returns NULL.

This is detected by Coccinelle semantic patch.
Signed-off-by: NWei Yongjun <weiyj.lk@gmail.com>
Reviewed-by: NGuenter Roeck <linux@roeck-us.net>
Signed-off-by: NGuenter Roeck <linux@roeck-us.net>
Signed-off-by: NWim Van Sebroeck <wim@iguana.be>