This post provides steps to enable the SSD option on disks/LUNs that are not detected as SSD by default.

vSphere 6:

In the Web Client browse to your host and select Manage - Storage - Storage Devices.
Now select the disk you want to mark as SSD and click the F icon.

Now we can see in the Web Client that the disk is marked as Flash.

vSphere 5:

Our LUN vmfs-coexist is currently tagged as a normal HDD

SSH into your ESXi host and issue esxcli storage nmp device list to note your hardware device id.

Now we need to create a new claim rule to mark the disk as Flash and the reclaim the device.

And that's it we can see the disk now as Flash