cpuidle: merge two if() statements for checking error cases

Two checks cpuidle_idle_call() cause the same error code to be
returned if they fail, so merge them for clarity.

[rjw: Changelog]
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>