brcmfmac: use brcmf_cfg80211_priv to interface with wl_cfg80211 code

The low-level driver part interfaces with wl_cfg80211 part using
brcmf_cfg80211_dev structure as handle. As brcmf_cfg80211_priv
is defined in interface it is more efficient to use that as handle
in function calls.

Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Reviewed-by: Hante Meuleman <meuleman@broadcom.com>
Reviewed-by: Franky (Zhenhui) Lin <frankyl@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>