net/mlx5e: RX, Generalise name of non-linear SKB head size

Make name more generic by dropping MPWRQ from it, as it will be
used also in Legacy RQ in a downstream patch.

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