iwlwifi: mvm: simplify iwl_mvm_stop_roc()

As pointed out by smatch, there's no need for a loop that always
immediately terminates. Use an if statement instead and while at
it clean up the mvmvif initialization.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>