cbus-retu-wdt: Fix bitfield access

An unsigned int pointer must not be casted to an unsigned
long pointer before use. Convert the bitfield to unsigned long
to fix this.
Also use clear_bit() in the release path.

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