cbus: retu: wdt: save dev in retu_wdt_dev

commit 6b8074b00d90b191227dc875b90b272c51f7d6eb
(cbus: Make retu watchdog behave like a standard
Linux watchdog) introduced struct retu_wdt_dev
with a field to save struct device *dev, but never
got the point of saving the pointer into that
structure.

Fix this very old bug so we can actually start
passing child device pointers to retu read/write
operations.

Signed-off-by: Felipe Balbi <balbi@ti.com>