staging: brcm80211: removed global var global_scb from softmac

Global variables are undesirable unless they are read only. Removed by
instead using an already defined Station Control Block variable in a
per-device structure.

Reported-by: Johannes Berg <johannes@sipsolutions.net>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Reviewed-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Franky Lin <frankyl@broadcom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>