cxgb4: Fix work request size calculation for loopback test

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>