x86/microcode/AMD: Change verify_patch()'s return value

Have it return 0 on success, positive value when the current patch
should be skipped and negative on error.

Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: x86@kernel.org
Link: https://lkml.kernel.org/r/20181107170218.7596-11-bp@alien8.de