platform/x86: intel-vbtn: Use acpi_evaluate_integer()

Use acpi_evaluate_integer() instead of open-coding it.

This is a preparation patch for adding a intel_vbtn_has_switches()
helper function.

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>