wifi: iwlwifi: mvm: refactor __iwl_mvm_assign_vif_chanctx()

wifi: iwlwifi: mvm: refactor __iwl_mvm_assign_vif_chanctx()

Since parts of the functionality of this function is going to be used
also by the MLD version of it, put in a separate function the parts
that are common for both MLD and non-MLD modes.
The common function will later be used in the MLD ops.

Signed-off-by: Miri Korenblit <miriam.rachel.korenblit@intel.com>
Signed-off-by: Gregory Greenman <gregory.greenman@intel.com>
Link: https://lore.kernel.org/r/20230314194113.844755701cac.I1c650718ad2381eabc38f4103c1aac67936a1ffc@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>