net/mlx5e: Use array_size() helper

Use array_size() helper to aid in 2-factor allocation instances.

Link: https://github.com/KSPP/linux/issues/160


Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>