platform/x86: asus_wmi: Fix return value of fan_boost_mode_store

Function fan_boost_mode_store returns 0 if store is successful,
this leads to infinite loop after any write to it's sysfs entry:

# echo 0 >/sys/devices/platform/asus-nb-wmi/fan_boost_mode

This command never ends, one CPU core is at 100% utilization.
This patch fixes this by returning size of written data.

Fixes: b096f626

 ("platform/x86: asus-wmi: Switch fan boost mode")
Signed-off-by: Leonid Maksymchuk <leonmaxx@gmail.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>