platform/x86: hp-wmi: Do not shadow error values

All the helper functions (i.e. hp_wmi_dock_state, hp_wmi_tablet_state,
...) using hp_wmi_perform_query to perform an HP WMI query shadow the
returned value in case of error.

We return -EINVAL only when the HP WMI query returns a positive value
(the specific error code) to not mix this up with the actual value
returned by the helper function.

Suggested-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Carlo Caione <carlo@endlessm.com>
Signed-off-by: Darren Hart (VMware) <dvhart@infradead.org>