ath9k: fix led naming

Fixed led device naming for the ath9k driver. Due to the
documentation of the led subsystem/class the naming should be
"devicename:colour:function" while not applying sections
should be left blank.

This should lead to e.g. "ath9k-%s::rx" instead of "ath9k-%s:rx".

Signed-off-by: Danny Kukawka <dkukawka@suse.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>