cxgbe(4): Fixes related to the knob that controls link autonegotiation.

• Do not leak the adapter lock in sysctl_autoneg.
• Accept only 0 or 1 as valid settings for autonegotiation.
• A fixed speed must be requested by the driver when autonegotiation is disabled otherwise the firmware will reject the l1cfg command. Use the top speed supported by the port for now.

MFC after: 3 days
Sponsored by: Chelsio Communications