drive: fix about (and df on a mount) for team drives - fixes #2288

Before this fix team drives would return the drive quota which is
incorrect and mis-leading.

Team drives don't appear to have an API for reading the bytes used or
the quota so we now return that the quota and usage are unknown.

backend/drive/drive.go

@@ -1057,6 +1057,10 @@ func (f *Fs) CleanUp() error {
 
 // About gets quota information
 func (f *Fs) About() (*fs.Usage, error) {
+	if f.isTeamDrive {
+		// Teamdrives don't appear to have a usage API so just return empty
+		return &fs.Usage{}, nil
+	}
 	var about *drive.About
 	var err error
 	err = f.pacer.Call(func() (bool, error) {