usb: device_next: uac2: Fix integer handling issues

Fix integer handling issues found out by Coverity Scan.

(cherry picked from commit 20983c7)

Original-Coverity-CID: 363729
Original-Coverity-CID: 363737
Original-Signed-off-by: Tomasz Moń <tomasz.mon@nordicsemi.no>
GitOrigin-RevId: 20983c7
Change-Id: I2205efa208db532a502b3bf4265c27273b591602
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/zephyr/+/5580447
Reviewed-by: Tristan Honscheid <honscheid@google.com>
Tested-by: ChromeOS Prod (Robot) <chromeos-ci-prod@chromeos-bot.iam.gserviceaccount.com>
Tested-by: Tristan Honscheid <honscheid@google.com>
Commit-Queue: Tristan Honscheid <honscheid@google.com>

subsys/usb/device_next/class/usbd_uac2.c

@@ -123,7 +123,8 @@ get_as_data_ep(struct usbd_class_data *const c_data, int as_idx)
 	const struct uac2_cfg *cfg = dev->config;
 	const struct usb_desc_header *desc = NULL;
 
-	if ((as_idx < cfg->num_ifaces) && cfg->ep_indexes[as_idx]) {
+	if ((as_idx >= 0) && (as_idx < cfg->num_ifaces) &&
+	    cfg->ep_indexes[as_idx]) {
 		desc = cfg->descriptors[cfg->ep_indexes[as_idx]];
 	}
 
@@ -344,6 +345,8 @@ static void write_explicit_feedback(struct usbd_class_data *const c_data,
 	int as_idx = terminal_to_as_interface(dev, terminal);
 	int ret;
 
+	__ASSERT_NO_MSG(as_idx >= 0);
+
 	buf = net_buf_alloc(&uac2_pool, K_NO_WAIT);
 	if (!buf) {
 		LOG_ERR("No buf for feedback");