b43: don't warn about no 5 GHz support on 2.4 GHz devices

This could be a bit confusing to see warning about lacking support for
5 GHz band if your device supports 2.4 GHz only.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>