ice: Fix the bytecount sent to netdev_tx_sent_queue

Currently if the driver does a TSO offload the bytecount sent to
netdev_tx_sent_queue will be incorrect. This is because in ice_tso we
overwrite the initial value that we set in ice_tx_map. This creates a
mismatch between the Tx and Tx clean flow. In the Tx clean flow we
calculate the bytecount (called total_bytes) as we clean the
descriptors so the value used in the Tx clean path is correct. Fix this
by using += in ice_tso instead of =. This fixes the mismatch in
bytecount mentioned above.

Signed-off-by: Brett Creeley <brett.creeley@intel.com>
Signed-off-by: Anirudh Venkataramanan <anirudh.venkataramanan@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>