net: mana: Set tx_packets to post gso processing packet count

Allow tx_packets and tx_bytes counter in the driver to represent
the packets transmitted post GSO processing.

Currently they are populated as bigger pre-GSO packets and bytes

Signed-off-by: Shradha Gupta <shradhagupta@linux.microsoft.com>
Reviewed-by: Haiyang Zhang <haiyangz@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>