tools/cpupower: fix error return value in cpupower_write_sysfs()

tools/cpupower: fix error return value in cpupower_write_sysfs()

The cpupower_write_sysfs() function currently returns -1 on
write failure, but the function signature indicates it should
return an unsigned int. Returning -1 from an unsigned function
results in a large positive value rather than indicating
an error condition.

Fix this by returning 0 on failure, which is more appropriate
for an unsigned return type and maintains consistency with typical
success/failure semantics where 0 indicates failure and non-zero
indicates success (bytes written).

Link: https://lore.kernel.org/r/20250828063000.803229-1-kaushlendra.kumar@intel.com
Signed-off-by: Kaushlendra Kumar <kaushlendra.kumar@intel.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>