atheros: introduce ath module containing common ath5k/ath9k/ar9170 code

This change creates a new module, ath.ko, which includes code that can
be shared between ath5k, ath9k and ar9170. For now, extract most of the ath9k
regulatory code so it can also be used in ath5k.

Signed-off-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>