ath5k: enable hardware encryption for WEP

This change re-enables hardware encryption for ath5k after setting up
mac80211 to handle the initialization vectors which happens to make it
work.  Add a module param (nohwcrypt) to optionally turn it off.

Signed-off-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>