x86/timers: Make recalibrate_cpu_khz() void

recalibrate_cpu_khz() is called from powernow K7 and Pentium 4/Xeon
CPU freq driver. It recalibrates cpu frequency in case of SMP = n
and doesn't need to return anything.

Mark it void, also remove the #else branch.

Signed-off-by: Dou Liyang <douly.fnst@cn.fujitsu.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/1500003247-17368-2-git-send-email-douly.fnst@cn.fujitsu.com