x86/cpu/centaur: Replace two-condition switch-case with an if statement

Use a normal if statements instead of a two-condition switch-case.

 [ bp: Massage commit message. ]

Signed-off-by: Tony W Wang-oc <TonyWWang-oc@zhaoxin.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/1599562666-31351-2-git-send-email-TonyWWang-oc@zhaoxin.com