hwmon/f71805f: Fix a race condition

I think I introduced a potential race condition bug with commit
51c997d8

. I didn't realize it
back then, but platform_device_put and platform_device_release
both appear to free the platform data associated with the device.
This makes an explicit kfree redundant at best, and maybe even
racy, as it might occur while someone still holds a reference
to the platform device.

Signed-off-by: Jean Delvare <khali@linux-fr.org>