Hello, please see this community document:
https://community.mellanox.com/docs/DOC-1460
Your adapter, switch and cable are all 56Gb capable.
With the show license command on the switch do you see a license supporting 56Gb?
Are you using a recent Mellanox OFED driver and a recent adapter firmware version?
ofed_info | head << to see OFED version
ethtool -i ethx <<to see fw version
Is the link clean (no discards or errors)?
On switch: show interface Ethernet 1/x
On server: ethtool -S ethx