brcmfmac: change struct brcmf_cfg80211_priv comments to kernel-doc

Small step to fix structure commenting using kernel-doc syntax.

Reviewed-by: Hante Meuleman <meuleman@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>