net/mlx5e: Fix calculated checksum offloads counters

Instead of calculating the offloads counters, count them explicitly.
The calculations done for these counters would result in bugs in some
cases, for example:
When running TCP traffic over a VXLAN tunnel with TSO enabled the following
counters would increase:
       tx_csum_partial: 1,333,284
       tx_csum_partial_inner: 29,286
       tx4_csum_partial_inner: 384
       tx7_csum_partial_inner: 8
       tx9_csum_partial_inner: 34
       tx10_csum_partial_inner: 26,807
       tx11_csum_partial_inner: 287
       tx12_csum_partial_inner: 27
       tx16_csum_partial_inner: 6
       tx25_csum_partial_inner: 1,733

Seems like tx_csum_partial increased out of nowhere.
The issue is in the following calculation in mlx5e_update_sw_counters:
s->tx_csum_partial = s->tx_packets - tx_offload_none - s->tx_csum_partial_inner;

While tx_packets increases by the number of GSO segments for each SKB,
tx_csum_partial_inner will only increase by one, resulting in wrong
tx_csum_partial counter.

Fixes: bfe6d8d1 ("net/mlx5e: Reorganize ethtool statistics")
Signed-off-by: Gal Pressman <galp@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>