brcmsmac: restructure info->control.sta handling as it is goning to be removed soon.

brcmsmac uses info->control.sta while doing ampdu aggregation. This patch
changes the usage of the structure info->control.sta, as it is going to be
removed soon from struct ieee80211_tx_info. This patch is a pre-requisit in
order to add transmission power control (TPC) to the mac80211 subsystem.

Suggested-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: Thomas Huehn <thomas@net.t-labs.tu-berlin.de>
Acked-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>