ath5k: added cfg80211 based rfkill support

This patch introduces initial rfkill support for the ath5k driver
based on rfkill support in the cfg80211 framework.
All rfkill related code is separated into newly created rfkill.c.

Changes to existing code are minimal:

* added a new data structure ath5k_rfkill to the ath5k_softc structure
* inserted calls to HW rfkill init/deinit routines
* ath5k_intr() has been extended to handle AR5K_INT_GPIO interrupts

Signed-off-by: John W. Linville <linville@tuxdriver.com>