platform/x86: intel-vbtn: Do not advertise switches to userspace if they are not there

Commit de9647ef ("platform/x86: intel-vbtn: Only activate tablet mode
switch on 2-in-1's") added a DMI chassis-type check to avoid accidentally
reporting SW_TABLET_MODE = 1 to userspace on laptops (specifically on the
Dell XPS 9360), to avoid e.g. userspace ignoring touchpad events because
userspace thought the device was in tablet-mode.

But if we are not getting the initial status of the switch because the
device does not have a tablet mode, then we really should not advertise
the presence of a tablet-mode switch to userspace at all, as userspace may
use the mere presence of this switch for certain heuristics.

Fixes: de9647ef

 ("platform/x86: intel-vbtn: Only activate tablet mode switch on 2-in-1's")
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>