b43: Fix for broken transmission

This patch fixes the transmission problems introduced by
commit f04b3787bbce4567e28069a9ec97dcd804626ac7

I'm not sure if the dummy read is really required.
The old code does it. I think it can't hurt and can possibly
fix some write posting problems (hardware bugs or whatever. Who knows).

Signed-off-by: Michael Buesch <mb@bu3sch.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>