x86/ucode/intel: Fix handling of microcode revision

x86/ucode/intel: Fix handling of microcode revision

For Intel microcode blobs, the revision field is signed (as documented in the
SDM) and negative revisions are used for pre-production/test microcode (not
documented publicly anywhere I can spot).

Adjust the revision checking to match the algorithm presented here:

  https://software.intel.com/security-software-guidance/best-practices/microcode-update-guidance

This treats pre-production microcode as always applicable, but also production
microcode having higher precedent than pre-production.  It is expected that
anyone using pre-production microcode knows what they are doing.

This is necessary to load production microcode on an SDP with pre-production
microcode embedded in firmware.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>