x86: clean up csum-wrappers_64.c some more

no code changed:

arch/x86/lib/csum-wrappers_64.o:
   text    data     bss     dec     hex filename
    839       0       0     839     347 csum-wrappers_64.o.before
    839       0       0     839     347 csum-wrappers_64.o.after
md5:
b31994226c33e0b52bef5a0e110b84b0  csum-wrappers_64.o.before.asm
b31994226c33e0b52bef5a0e110b84b0  csum-wrappers_64.o.after.asm

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>