Fix the following coccicheck warning:

drivers/net/wireless/broadcom/brcm80211/brcmsmac/stf.c:309:5-13:
Unneeded variable: "ret_code". Return "0" on line 328
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200413082126.22572-1-yanaijie@huawei.com