staging: brcm80211: only allow one call to add_interface callback

The driver only supports one network interface to be associated
with it. Only for the first call it does actually something. For
subsequent calls it does return -ENODEV error code.

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