watchdog: dw: remove useless pr_fmt

When switch to watchdog infrastructure, pr_* usage is removed, so
there's no any users of the pr_fmt, remove it.

Signed-off-by: Jisheng Zhang <Jisheng.Zhang@synaptics.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@linux-watchdog.org>