iwlwifi: fix TX tracer

The TX tracing code copies with the wrong length,
which will typically copy too little data. Fix
this by using the correct length variable.

Cc: stable@kernel.org [2.6.32+]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>