mlxsw: Remove 56G speed support

Commit 275e928f ("mlxsw: spectrum: Prevent force of 56G") prevented
the driver from setting a speed of 56G when auto-negotiation is off.
This is the only speed supported by mlxsw that cannot be set when
auto-negotiation is off, which makes it difficult to write generic
tests.

Further, the speed is not supported by newer ASICs such as Spectrum-2
and to the best of our knowledge it is not used by current users.

Therefore, remove 56G support from mlxsw.

Signed-off-by: Amit Cohen <amitc@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>