x86: apic - fix unused vars warning in calibrate_APIC_clock

If we don't have CONFIG_X86_PM_TIMER=y compiler warns about
unused variables. Move PM timer based calibration into a
separate function and make the code cleaner and the compiler
happy as well.

Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>