FogBugz #532583-2: hwmon: Fix buffer calls for SDL compliance.

commit  ac2b8b68a1e2d58bc5520349dac72f659750fdc4 from
https://github.com/altera-opensource/linux-socfpga.git



The buffer calls need to be more secure so pass the buffer size
in the sprintf functions. Limit buffer size to 5.

Signed-off-by: Thor Thayer <thor.thayer@linux.intel.com>
Signed-off-by: Meng Li <Meng.Li@windriver.com>