brcmfmac: get rid of struct brcmf_cfg80211_info::link_up attribute

This attribute indicates successful IBSS or AP connection has been
established. However, this no longer works for virtual interfaces.
As it turns out this attribute is identical to the CONNECTED bit
in struct brcmf_cfg80211_vif::sme_state. This patch removes the
attribute and rework some functions relying on it.

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>