ath9k: Add a debugfs interface for controlling virtual wiphys

debugfs ath9k/phy#/wiphy can be used to show the current list of
virtual wiphys and to add/remove virtual wiphys. Eventually, this
interface could be replaced with a cfg80211/nl80211 command that is
passed through mac80211.

For example:
# cat /debug/ath9k/phy0/wiphy
primary: phy0
# echo add > /debug/ath9k/phy0/wiphy
# cat /debug/ath9k/phy0/wiphy
primary: phy0
secondary: phy1
# echo del=phy1 > /debug/ath9k/phy0/wiphy
# cat /debug/ath9k/phy0/wiphy
primary: phy0

In addition, following commands can be used to test pausing and
unpausing of the virtual wiphys:
pause=phy1
unpause=phy1
select=phy1
(select pauses and unpauses wiphys automatically based on channel)
schedule=500
(set wiphy scheduling interval in msec; 0 = disable; default value: 500)

Signed-off-by: Jouni Malinen <jouni.malinen@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>