net/mlx5e: Make RSS indirection table size a constant

The indirection table size was defined by a variable that
was actually assigned a constant value.
Since we do not have any forseen intension to make it configurable
we simply made it a constant.

We also limit the number of channels such that the RSS indirection
table could always populate all RX rings.

Signed-off-by: Achiad Shochat <achiad@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>