usb: host: xhci-plat: Do not define 'struct acpi_device_id' when !CONFIG_ACPI

usb: host: xhci-plat: Do not define 'struct acpi_device_id' when !CONFIG_ACPI

Since ACPI_PTR() is used to NULLify the value when !CONFIG_ACPI,
struct usb_xhci_acpi_match becomes defined by unused.

Fixes the following W=1 kernel build warning(s):

 drivers/usb/host/xhci-plat.c:457:36: warning: ‘usb_xhci_acpi_match’ defined but not used [-Wunused-const-variable=]
 457 | static const struct acpi_device_id usb_xhci_acpi_match[] = {
 | ^~~~~~~~~~~~~~~~~~~

Cc: Mathias Nyman <mathias.nyman@intel.com>
Cc: Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20200706133341.476881-33-lee.jones@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>