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