ath9k: fix spectral scan endless mode on AR9002

There was a copy+paste error in ar9002 for the endless spectral mode,
fix that.

Signed-off-by: Simon Wunderlich <siwu@hrz.tu-chemnitz.de>
Signed-off-by: Mathias Kretschmer <mathias.kretschmer@fokus.fraunhofer.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>