watchdog: mpc8xxx_wdt: Convert to use device managed functions and other improvements

Use device managed functions to simplify error handling, reduce
source code size, improve readability, and reduce the likelyhood of bugs.
Other improvements as listed below.

The conversion was done automatically with coccinelle using the
following semantic patches. The semantic patches and the scripts
used to generate this commit log are available at
https://github.com/groeck/coccinelle-patches



- Drop assignments to otherwise unused variables
- Drop empty remove function
  This also drops the inaccurate message on remove; the driver won't be
  removed if the hardware watchdog is running, no matter if the watchdog
  device is open or not.
- Use local variable 'struct device *dev' consistently
- Use devm_watchdog_register_driver() to register watchdog device

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>