watchdog: at32ap700x_wdt: register misc device last in probe() function

This patch reworks the probe() function in the at32ap700x_wdt driver, this to
make sure the miscdev is properly initialized and the driver is ready to be
accessed.

Reported-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Hans-Christian Egtvedt <hans-christian.egtvedt@atmel.com>
Signed-off-by: Wim Van sebroeck <wim@iguana.be>