bonding: 3ad: Print an error for unknown speeds

The bond driver needs to be patched to support new ethtool speeds.
Currently it emits a single warning [1] when it encounters an unknown
speed. As evident by the two previous patches, this is not explicit
enough. Instead, promote it to an error.

[1]
bond10: (slave swp1): unknown ethtool speed (200000) for port 1 (set it to 0)

v2:
* Use pr_err_once() instead of WARN_ONCE()

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Nikolay Aleksandrov <nikolay@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>