PM / OPP: Simplify _opp_set_availability()

As we don't use RCU locking anymore, there is no need to replace an
earlier OPP node with a new one. Just update the existing one.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>