x86/platform/UV: Clean up the UV APIC code

Make it more readable.
Acked-by: NThomas Gleixner <tglx@linutronix.de>
Cc: Dimitri Sivanich <sivanich@hpe.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Travis <travis@sgi.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20170114082612.GA27842@gmail.comSigned-off-by: NIngo Molnar <mingo@kernel.org>