cpufreq: interactive: Add error checking on sysfs interfaces

This adds better error checking on tunable parameters on sysfs
interfaces.  Also fixes return value from these functions, previously
on success they would return 0 which would cause a infinite loop.

Change-Id: Ic05038492166f8673d007202092471f98a2f0dfa
Signed-off-by: Allen Martin <amartin@nvidia.com>