brcmfmac: remove mode from struct brcmf_cfg80211_conf

The mode should be stored and used per virtual interface. Remove
the mode from device global structure and rework the code to use
the mode from brcmf_cfg80211_vif.

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>