net/mlx5e: Initialize on stack link modes bitmap

Initialize link modes bitmap on stack before using it, otherwise the
outcome of ethtool set link ksettings might have unexpected values.

Fixes: 4b95840a ("net/mlx5e: Fix matching of speed to PRM link modes")
Signed-off-by: Aya Levin <ayal@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>