platform/x86: intel-vbtn: Fix probe failure on devices with only switches

On some devices the INT33D6 vbtn device is only used to report
tablet-mode / docked status (switches) and there are no vbtn managed
buttons.

On these devices there is no VBDL object.

Move the VBDL check to a intel_vbtn_has_buttons() helper and only exit
from intel_vbtn_probe() with -ENODEV when there are both no buttons and
no switches. Also only report the buttons being present to userspace if
the has_buttons check has succeeded.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>