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: Irina Tirdea <irina.tirdea@intel.com>
Reported-by: Hartmut Knaack <knaack.h@gmx.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>