brcmsmac: make brcms_c_set_mac() void

Fix the following coccicheck warning:

drivers/net/wireless/broadcom/brcm80211/brcmsmac/main.c:3773:5-8:
Unneeded variable: "err". Return "0" on line 3781

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Jason Yan <yanaijie@huawei.com>
Acked-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200410090817.26883-1-yanaijie@huawei.com