brcmfmac: introduce brcmf_cfg80211_vif structure

This patch introduces the brcmf_cfg80211_vif structure which is
used to keep track of multiple virtual interfaces in the driver.

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