ath5k: move external function definitions to a header file

Johannes pointed out the mess of external function prototypes in the
mac80211-ops.c file.  Woe to anyone who changes these functions...

Signed-off-by: Bob Copeland <me@bobcopeland.com>
Cc: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>