KVM: x86: Simplify msr_io()

As of commit bccf2150 ("KVM: Per-vcpu inodes"), __msr_io() doesn't
return a negative value. Remove unnecessary checks.

Signed-off-by: Michal Luczaj <mhal@rbox.co>
Link: https://lore.kernel.org/r/20230107001256.2365304-7-mhal@rbox.co


[sean: call out commit which left behind the unnecessary check]
Signed-off-by: Sean Christopherson <seanjc@google.com>