net/mlx5e: Refactor inline header size calculation in the TX path

As preparation for the next patch, don't increase ihs to calculate
ds_cnt and then decrease it, but rather calculate the intermediate value
temporarily. This code has the same amount of arithmetic operations, but
now allows to split out ds_cnt calculation, which will be performed in
the next patch.

Signed-off-by: Maxim Mikityanskiy <maximmi@mellanox.com>
Reviewed-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>