libertas_tf: constify usb_device_id

usb_device_id are not supposed to change at runtime. All functions
working with usb_device_id provided by <linux/usb.h> work with
const usb_device_id. So mark the non-const structs as const.
Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>

drivers/net/wireless/marvell/libertas_tf/if_usb.c

@@ -31,7 +31,7 @@ module_param_named(fw_name, lbtf_fw_name, charp, 0644);
 
 MODULE_FIRMWARE("lbtf_usb.bin");
 
-static struct usb_device_id if_usb_table[] = {
+static const struct usb_device_id if_usb_table[] = {
 	/* Enter the device signature inside */
 	{ USB_DEVICE(0x1286, 0x2001) },
 	{ USB_DEVICE(0x05a3, 0x8388) },