cpuidle: use drv instead of cpuidle_driver in show_current_driver()

Instances of "struct cpuidle_driver *" are consistently named as "drv"
in the cpuidle core except in show_current_driver().

Make that function use variable naming consistent with the rest of the
code.

[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>