iwlwifi: enable packet injection for iwlagn

Handle station IDs of transmitted packets when in monitor mode, and
remove the various anti-injection checks from the iwl4965 driver.
This makes injection work on iwl4965 and iwl5000. Tested on both cards.

Note: To inject management frames with encryption, HW crypto support
must be disabled using the "swcrypto=1" modparam (or "swcrypto50=1"
for iwl5000). Otherwise most management frames won't be transmitted.

Signed-off-by: Gábor Stefanik <netrolller.3d@gmail.com>
Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>