x86/microcode/amd: Fix two -Wunused-but-set-variable warnings

The dummy variable is the high part of the microcode revision MSR which
is defined as reserved. Mark it unused so that W=1 builds don't trigger
the above warning.

No functional changes.

Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: x86@kernel.org
Link: https://lkml.kernel.org/r/20190928162559.26294-1-bp@alien8.de