x86/platform/UV: Mark tsc_check_sync as an init function

Fix build problem:

>> WARNING: vmlinux.o(.text+0x4223a): Section mismatch in
   reference from the function uv_tsc_check_sync() to the function
   .init.text:uv_early_read_mmr() The function uv_tsc_check_sync()
   references the function __init uv_early_read_mmr().  This is often
   because uv_tsc_check_sync lacks a __init

Signed-off-by: Mike Travis <mike.travis@hpe.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Prarit Bhargava <prarit@redhat.com>
Cc: Dimitri Sivanich <dimitri.sivanich@hpe.com>
Cc: Russ Anderson <russ.anderson@hpe.com>
Cc: Andrew Banman <andrew.banman@hpe.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Bin Gao <bin.gao@linux.intel.com>
Link: https://lkml.kernel.org/r/20171023191841.985614692@stormcage.americas.sgi.com