x86_64: remove nested irq disables

setup_APIC_timer disables interrupts anyway. So no need to do the same
in setup_boot_APIC_clock and setup_secondary_APIC_clock. Disable
interrupts explicit in the calibration code.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Chris Wright <chrisw@sous-sol.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>