x86/microcode: Avoid undefined behaviour from signed integer overflow

x86/microcode: Avoid undefined behaviour from signed integer overflow

The checksums should be calculated using unsigned 32bit integers, as they are
intended to overflow and end at 0.  Replace some other signed integers with
unsigned ones, to avoid mixed-sign comparisons.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Kevin Tian <kevin.tian@intel.com>