rndis_wlan: use power save only for BCM4320b

BCM4320a breaks when enabling power save (bug 29732). So disable power save
for anything but BCM4320b that is known to work.

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