KVM: selftests: Fix a typo in x86-64's kvm_get_cpu_address_width()

Fix a == vs. = typo in kvm_get_cpu_address_width() that results in
@pa_bits being left unset if the CPU doesn't support enumerating its
MAX_PHY_ADDR.  Flagged by clang's unusued-value warning.

lib/x86_64/processor.c:1034:51: warning: expression result unused [-Wunused-value]
                *pa_bits == kvm_cpu_has(X86_FEATURE_PAE) ? 36 : 32;

Fixes: 3bd39635 ("KVM: selftests: Add X86_FEATURE_PAE and use it calc "fallback" MAXPHYADDR")
Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20221213001653.3852042-6-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>