watchdog: imgpdc: Unregister restart handler on remove

Commit c631f200 ("watchdog: imgpdc: Add reboot support") introduced
a restart handler but forgot to unregister it on driver removal. Fix it.

Fixes: c631f200 ("watchdog: imgpdc: Add reboot support")
Reported-by: Ariel D'Alessandro <ariel@vanguardiasur.com.ar>
Signed-off-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>