iwlwifi: mvm: refactor iwl_mvm_switch_vif_chanctx to support other modes

Currently we only support the CHANCTX_SWMODE_SWAP_CONTEXTS mode, but
we need to support other modes as well.  Spin a new function off in
order to make it easier to support other modes.

Signed-off-by: Luciano Coelho <luciano.coelho@intel.com>