platform/x86: hp-wmi: Do not shadow errors in sysfs show functions

The new hp_wmi_read_int function returns a negative value in case of
error, pass this on directly rather than always replacing it with
-EINVAL.

Signed-off-by: Darren Hart (VMware) <dvhart@infradead.org>
Tested-by: Carlo Caione <carlo@endlessm.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>