net/mlx5e: Tx, Strict the room needed for SQ edge NOPs

We use NOPs to populate the WQ fragment edge if the WQE does not fit
in frag, to avoid WQEs crossing a page boundary (or wrap-around the WQ).

The upper bound on the needed number of NOPs is one WQEBB less than
the largest possible WQE, for otherwise the WQE would certainly fit.

Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>