iwl3945: use iwl_rb_status

This patch makes use of iwl_rb_status also in 3945. The structure
for 3945 is not the same but since only closed_rb_num filed is used
in both cases there is no reason to duplicate it.

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>