The adapter must have a PTP/HW timestamping activation key installed. There is currently no way to add or check the activation key from the ESXi host.
To add/check for the PTP activation key on a Linux server, use the sfkey utility from the Solarflare Linux Utilities package (SF-107601-LS):
# sfkey
p1p1,p1p2: 000F53650950
Product name Solarflare XtremeScale X2522-25G Adapter
Serial number 252221402110182318500104
Installed keys Plus
p2p1,p2p2: 000F535CFFA0
Product name Solarflare Flareon Ultra 8000 Series 10G Adapter
Serial number 852200213000173117100590
Installed keys Onload, PTP
An adapter with a Plus key includes the PTP key.