x86/tsc_msr: Remove irqoff around MSR-based TSC enumeration

Remove the irqoff/irqon around MSR-based TSC enumeration,
as it is not necessary.

Also rename: try_msr_calibrate_tsc() to cpu_khz_from_msr(),
as that better describes what the routine does.

Signed-off-by: Len Brown <len.brown@intel.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/a6b5c3ecd3b068175d2309599ab28163fc34215e.1466138954.git.len.brown@intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>