net/mlx5e: Initialize link speed to zero

mlx5e_port_max_linkspeed does not guarantee value assignment for speed.
Avoid cases where link_speed might be used uninitialized. In case
mlx5e_port_max_linkspeed fails, a default link speed of 50000 will be
used for the calculations.

Fixes: 3f6d08d1

 ("net/mlx5e: Add RSS support for hairpin")
Signed-off-by: Roy Novich <royno@nvidia.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Reviewed-by: Aya Levin <ayal@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>