net/mlx5e: Correctly handle RSS indirection table when changing number of channels

Upon changing num_channels, reset the RSS indirection table to
match the new value.

Fixes: 2d75b2bc ('net/mlx5e: Add ethtool RSS configuration options')
Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>