iwlagn: verify mutex held for sync commands

Emmanuel noticed that there's no explicit checking
that prevents the driver from attempting to issue
multiple synchronous commands at the same time and
wrote a patch to check. However, his patch warns
only if a collision actually happened, an unlikely
thing since the driver mutex should be held for
synchronous command submissions.

So instead of checking that a collision happened
add a check that the mutex is held which ensures
that collisions can't happen.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>