cpupower tools: Fix issues with sysfs_topology_read_file

Fix a variety of issues with sysfs_topology_read_file:
* The return value of sysfs_topology_read_file function was not properly
  being checked for failure.
* The function was reading int valued sysfs variables and then returning
  their value. So, even if a function was trying to check the return value
  of this function, a caller would not be able to tell an failure code apart
  from reading a negative value. This also conflicted with the comment on the
  function which said that a return value of 0 indicated success.
* The function was parsing int valued sysfs values with strtoul instead of
  strtol.
* The function was non-static even though it was only used in the
  file it was declared in.

Signed-off-by: Palmer Cox <p@lmercox.com>
Signed-off-by: Thomas Renninger <trenn@suse.de>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>