iwlwifi: add option to disable 5GHz band

There are various problems happened on 5GHz band not observed on
2.4 GHz (microcode errors, queue stuck, etc... ) . Also roaming
between 5GHz AP and 2GHz does not work very well. To workaround
the problems add option to disable 5GHz support. This will help
on environments where APs are dual-band, and devices will not try
to associate on band where issues happen.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>