usb: host: ehci-platform: Do not define 'struct acpi_device_id' when !CONFIG_ACPI

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

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

 drivers/usb/host/ehci-platform.c:478:36: warning: ‘ehci_acpi_match’ defined but not used [-Wunused-const-variable=]
 478 | static const struct acpi_device_id ehci_acpi_match[] = {
 | ^~~~~~~~~~~~~~~

Cc: Tony Prisk <linux@prisktech.co.nz>
Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: Philipp Zabel <p.zabel@pengutronix.de>
Cc: Steven Brown <sbrown@cortland.com>
Cc: Hauke Mehrtens <hauke@hauke-m.de>
Cc: de Goede <hdegoede@redhat.com>
Cc: Michael Buesch <m@bues.ch>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20200706133341.476881-29-lee.jones@linaro.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/host/ehci-platform.c

@@ -475,11 +475,13 @@ static const struct of_device_id vt8500_ehci_ids[] = {
 };
 MODULE_DEVICE_TABLE(of, vt8500_ehci_ids);
 
+#ifdef CONFIG_ACPI
 static const struct acpi_device_id ehci_acpi_match[] = {
 	{ "PNP0D20", 0 }, /* EHCI controller without debug */
 	{ }
 };
 MODULE_DEVICE_TABLE(acpi, ehci_acpi_match);
+#endif
 
 static const struct platform_device_id ehci_platform_table[] = {
 	{ "ehci-platform", 0 },