ath9k_hw: fix dual band assumption for XB113

The XB113 cards are single band, 5 GHz-only, but the
default settings were configured to assume it was dual
band. Users of these cards then would see 2.4 GHz channels
but you would never get any scan results from these channels
given that the radio is not present.

Cc: stable@kernel.org
Cc: Fiona Cain <Fiona.Cain@atheros.com>
Cc: Ray Li <ray.li@greenwavereality.com>
Cc: Kathy Giori <kathy.giori@atheros.com>
Cc: Aeolus Yang <aeolus.yang@atheros.com>
Cc: Dan Friedman <dan.friedman@atheros.com>
Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>