cbus-retu-wdt: Remove unused nonstandard interfaces

This removes nonstandard interfaces from the WDT driver.
/dev/watchdog should be used to access the watchdog, only.

It removes the sysfs interface.
It also removes the "counter_param" module parameter, which
currently is broken anyway (doesn't work, because the value
is overridden right away).

Signed-off-by: Michael Buesch <mb@bu3sch.de>
Signed-off-by: Tony Lindgren <tony@atomide.com>