i40e: fix Tx descriptors reset

i40e: fix Tx descriptors reset

Fix the descriptor initialization loop, so that it initializes
the i40e_tx_desc::cmd_type_offset_bsz for the correct index
into the tx_ring array.

Previously it would use the index once to initialize the txd
local variable, then again when setting cmd_type_offset_bsz.

Signed-off-by: Jim Harris <james.r.harris@intel.com>
Acked-by: Helin Zhang <helin.zhang@intel.com>