ath9k: Properly initialize channel table for 2GHz

ath9k channel table for 2Ghz does not seems to initialize the 'band'
parameter.Though it does not seems to cause any visible issue it looks
odd when we initialize the 'band' parameter for 5Ghz channel table while
not so for 2Ghz.

Signed-off-by: Mohammed Shafi Shajakhan <mshajakhan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>