Support HGST SSD400M SAS drives

[pruiz]

Hi,

While working with the aforementioned SAS/SSD drives I've found the length field reported by the drive to a discovery0 request to be incorrect. Instead of reporting the full length of the discovery0 header + feature pages. It only reports the space occupied by feature pages (assuming the discovery0 header size is fixed and can be accounted on it's own, I guess).

By reading the spec (see: https://trustedcomputinggroup.org/wp-content/uploads/TCG_Storage_Architecture_Core_Spec_v2.01_r1.00.pdf), the length field is described as:

3.3.6.2.1 Length of parameter data

Indicates the total number of bytes that are valid in the Level 0 Discovery header and all of the feature
descriptors returned, not including this field.

However, as stated, this drives (incorrectly?) return a length field not accounting for discovery0 header itself. So, this patch, overcomes such issue, by assuming a length field smaller than the discovery0 header size, should not be accounting for the header. So we keep parsing discovery0 response by as much as such length bytes of feature pages.