iwlwifi: mvm: look for the first supported channel when add/remove phy ctxt

Can't rely that band 2.4 is always supported by the NIC and use the
first channel in this band for the phy ctxt.
Instead, look for the first channel in the first band that is supported

Signed-off-by: Tova Mussai <tova.mussai@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>