cpuidle: sysfs: Accept governor name with 15 characters

CPUIDLE_NAME_LEN is 16, so it's possible to accept governor name
with 15 characters, but now store_current_governor() rejects
governor name with 15 characters as it returns -EINVAL if count
equals CPUIDLE_NAME_LEN.

Refactor the code to accept such case and simplify the code.

Signed-off-by: Hanjun Guo <guohanjun@huawei.com>
Reviewed-by: Doug Smythies <dsmythies@telus.net>
Tested-by: Doug Smythies <dsmythies@telus.net>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>