ath5k: ath5k_copy_channels() was not setting the channel band

ath5k_copy_channels() wasn't setting the channel's band so all
driver channels had a 2GHz band set. Lets set this.

Changes-licensed-under: 3-Clause-BSD

Signed-off-by: Luis R. Rodriguez <mcgrof@winlab.rutgers.edu>
Signed-off-by: John W. Linville <linville@tuxdriver.com>