x86, microcode, AMD: Simplify install_equiv_cpu_table

There's no need to memcpy the ucode header in order to look at it only
in this function - use the original buffer instead. Also, fix return
type semantics by returning a negative value on error and a positive
otherwise.

Signed-off-by: Borislav Petkov <borislav.petkov@amd.com>
Acked-by: Andreas Herrmann <Andreas.Herrmann3@amd.com>