iwlwifi: return NETDEV_TX_OK from _tx ops

be consistent with mac80211 drivers and return correct return code.
NETDEV_TX_OK is 0, but we need to be consistent wrt formatting amongst
implementations

re: http://marc.info/?l=linux-wireless&m=123119327419865&w=2

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