Add new enum in regulatory to hold all current phymodes.
Add a new API to get max allowed phymode which is not more than the
user provided phymode.
This change simplifies the phymode selection process when starting
or updating the parameters of a vdev.
Change-Id: I12c15e357b80faf2f5272800e55179f02755a263
CRs-fixed: 2658290