watchdog: gpio_wdt: Fix missing platform_set_drvdata() in gpio_wdt_probe()

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: Wei Yongjun <weiyj.lk@gmail.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>