brcmfmac: include linux/atomic.h

brcmfmac uses atomic_or() and other atomic_* functions, but does not
include linux/atomic.h. This file gets included by some other header
file so this normally does not cause problems.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Acked-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>