x86, microcode_amd: cleanup, mark request_microcode_user() as unsupported

(1) mark mc_size in generic_load_microcode() as unitialized_var to avoid
     gcc's (false) warning;

(2) mark request_microcode_user() as unsupported. The required changes
    can be added later. Note, we don't break any user-space interfaces
    here, as there were no kernels with support for AMD-specific ucode
    update yet. The ucode has to be updated via 'firmware'.

Signed-off-by: Dmitry Adamushko <dmitry.adamushko@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>