ACER: Fix up sparse warning

drivers/platform/x86/acer-wmi.c:1836:18: sparse: Using plain integer as NULL pointer

drivers/platform/x86/acer-wmi.c:1836:
  1833
  1834          BUG_ON(!name || !ah);
  1835
> 1836          handle = 0;
  1837          status = acpi_get_devices(prop, acer_wmi_get_handle_cb,
  1838                                          (void *)name, &handle);
  1839

Reported-by: Fengguang Wu <wfg@linux.intel.com>

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Matthew Garrett <mjg59@srcf.ucam.org>
Cc: Fengguang Wu <wfg@linux.intel.com>
Cc: joeyli <jlee@suse.com>
Signed-off-by: Matthew Garrett <mjg@redhat.com>