Work request used for sending loopback packet needs to add
the firmware work request only once. So, fix by using
correct structure size.

Fixes: 7235ffae ("cxgb4: add loopback ethtool self-test")
Signed-off-by: Ganji Aravind <ganji.aravind@chelsio.com>
Reviewed-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>