brcmfmac: rework .get_station() callback

The .get_station() cfg80211 callback is used in several scenarios. In
managed mode it can obtain information about the access-point and its
BSS parameters. In managed mode it can also obtain information about
TDLS peers. In AP mode it can obtain information about connected
clients.

Reviewed-by: Hante Meuleman <meuleman@broadcom.com>
Reviewed-by: Daniel (Deognyoun) Kim <dekim@broadcom.com>
Reviewed-by: Franky (Zhenhui) Lin <frankyl@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>