iwlwifi: move rx queue read pointer into rxq

This patch moves rx status/read registers into
iwl_rx_queue structures. This solution is more memory
hungry but is more structured and provides needed RX/TX
separation

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>