ath9k: make rfkill configurable

When the platform doesn't have rfkill support, i.e. nothing
is connected to the rfkill GPIO, there's little value in
polling the GPIO. Add a Kconfig option to allow disabling
the polling in ath9k.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>