wifi: iwlwifi: mvm: fix mvmtxq->stopped handling

[ Upstream commit b58e3d43 ]

This could race if the queue is redirected while full, then
the flushing internally would start it while it's not yet
usable again. Fix it by using two state bits instead of just
one.

Reviewed-by: Benjamin Berg <benjamin.berg@intel.com>
Tested-by: Jose Ignacio Tornos Martinez <jtornosm@redhat.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>