cxgb4: Don't disallow turning off auto-negotiation

For {1, 10, 40} Gb/s. Prohibiting turning off autonegotiation isn't anywhere
in the standard.

Signed-off-by: Hariprasad Shenai <hariprasad@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>