net/mlx5e: E-switch, Fix rate calculation division

do_div() returns reminder, while cited patch wanted to use
quotient.
Fix it by using quotient.

Fixes: 0e22bfb7 ("net/mlx5e: E-switch, Fix rate calculation for overflow")
Signed-off-by: Parav Pandit <parav@nvidia.com>
Signed-off-by: Maor Dickman <maord@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>