Which type of cable you are using? what is the vendor and part number?
Cisco are not locking the cable on it's end?
you configured the port on the Mellanox side to 10G?
Saw this on their forums.
This answered my question. They already used their 10GbE uplinks the reason Mellanox auto negotiated to 1GbE speed.
Single Supervisor Mode
In single supervisor mode, WS-X45-SUP-7L-E supports the uplink configuration of at most either two 10-Gigabit or four 1-Gigabit ports
*Posting external link for future connectivity references.