Using SMI to check speed of ethernet phy
Yes, the basic status register indicates the PHY's capabilities.  The register map shows that those bits are read-only.  

The datasheet does tell you how to read the actual link speed:


  Once auto-negotiation has completed, information about the resolved link can be passed back to the controller via the Serial Management Interface (SMI). The results of the negotiation process are reflected in the Speed Indication bits of 
  the PHY Special Control/Status Register, as well as in the Auto Negotiation Link Partner Ability Register. The auto-negotiation protocol is a purely physical layer activity and proceeds independently of the MAC controller.


So you could take the highest common capability between what your PHY advertises and what its partner advertises, but it would be simpler and more reliable to look at the actual link status: