When setting the activity period, the value introduced by
the user in sysfs is not checked for validity.

Add a boundary check so that only allowed values are
reported as successfully written to device.
Signed-off-by: NIrina Tirdea <irina.tirdea@intel.com>
Reported-by: NHartmut Knaack <knaack.h@gmx.de>
Signed-off-by: NJonathan Cameron <jic23@kernel.org>