x86, clang: Breakage with thinkpad_acpi

lll-project (llvm-linux with clang)

  CC [M]  drivers/platform/x86/thinkpad_acpi.o
clang: warning: argument unused during compilation: '-freg-struct-return'
clang: warning: argument unused during compilation: '-mpreferred-stack-boundary=2'
clang: warning: argument unused during compilation: '-fno-optimize-sibling-calls'
  CC      drivers/pci/pcie/aer/aerdrv_acpi.o
clang: warning: argument unused during compilation: '-freg-struct-return'
clang: warning: argument unused during compilation: '-mpreferred-stack-boundary=2'
clang: warning: argument unused during compilation: '-fno-optimize-sibling-calls'
  CC [M]  drivers/pci/hotplug/shpchp_hpc.o
clang: warning: argument unused during compilation: '-freg-struct-return'
clang: warning: argument unused during compilation: '-mpreferred-stack-boundary=2'
clang: warning: argument unused during compilation: '-fno-optimize-sibling-calls'
drivers/platform/x86/thinkpad_acpi.c:2400:4: error: implicit declaration of function 'issue_volchange' is invalid in C99 [-Wimplicit-function-declaration]
                        issue_volchange(oldn->volume_level, newn->volume_level);
                        ^
drivers/platform/x86/thinkpad_acpi.c:6030:37: warning: comparison of unsigned expression < 0 is always false [-Wtautological-compare]
        if (value > bright_maxlvl || value < 0)
                                     ~~~~~ ^ ~
1 warning and 1 error generated.