staging: brcm80211: removed redundant wlc->cfg struct member

After initialization ('attach'), this struct member always pointed at
the same memory as wlc->bsscfg.

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>