x86/efi: fix a -Wtype-limits compilation warning

Compiling a kernel with W=1 generates this warning,

arch/x86/platform/efi/quirks.c:731:16: warning: comparison of unsigned
expression >= 0 is always true [-Wtype-limits]

Fixes: 3425d934 ("efi/x86: Handle page faults occurring while running ...")
Signed-off-by: Qian Cai <cai@lca.pw>
Acked-by: "Prakhya, Sai Praneeth" <sai.praneeth.prakhya@intel.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>