brcmfmac: get rid of void pointer in struct brcmf_cfg80211_priv

Field 'pub' in struct brcmf_cfg80211_priv was types as void pointer
and filled with parameter passed in brcmf_cfg80211_attach(). This
patch makes the type specific, ie. struct brcmf_pub.

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>