x86, microcode, AMD: Simplify get_next_ucode

Do not copy the section header but look at it directly through the
pointer. Also, make it return a ptr to a ucode header directly
thus dropping a bunch of unneeded casts. Finally, simplify
generic_load_microcode(), while at it.

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