rndis_wlan: fix support for bcm4320a

Old variant of bcm4320 doesn't handle setting configuration parameters
correctly. One symptom is that MAC gets partially overwritten when any
config parameters are set. This patch disables config-params for bcm4320a
(and generic rndis-wlan).

Signed-off-by: Jussi Kivilinna <jussi.kivilinna@mbnet.fi>
Signed-off-by: John W. Linville <linville@tuxdriver.com>