brcmfmac: check return value of driver_for_each_device()

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

drivers/net/wireless/broadcom/brcm80211/brcmfmac/usb.c:1576:6: warning:
 variable 'ret' set but not used [-Wunused-but-set-variable]
  1576 |  int ret;
       |      ^~~

driver_for_each_device() has been declared with __must_check, so the
return value should be checked.
Signed-off-by: Zhang Changzhong <zhangchangzhong@huawei.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/1600481191-14250-1-git-send-email-zhangchangzhong@huawei.com

drivers/net/wireless/broadcom/brcm80211/brcmfmac/usb.c

@@ -1578,6 +1578,9 @@ void brcmf_usb_exit(void)
 	brcmf_dbg(USB, "Enter\n");
 	ret = driver_for_each_device(drv, NULL, NULL,
 				     brcmf_usb_reset_device);
+	if (ret)
+		brcmf_err("failed to reset all usb devices %d\n", ret);
+
 	usb_deregister(&brcmf_usbdrvr);
 }