iwlwifi: mvm: finalize on post_switch instead of unassign

Now that mac80211 waits for the first beacon on the new channel before
calling post_channel_switch, the reconfiguration of the firmware
should be done in the post_channel_switch operation instead of when
assigning the vif to the new context.

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