swiotlb: Mark max_segment with static keyword

Sparse is not happy about max_segment declaration:

  CHECK   kernel/dma/swiotlb.c
  kernel/dma/swiotlb.c:96:14: warning: symbol 'max_segment' was not declared. Should it be static?

Mark it static as suggested.
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

kernel/dma/swiotlb.c

@@ -93,7 +93,7 @@ static unsigned int io_tlb_index;
  * Max segment that we can provide which (if pages are contingous) will
  * not be bounced (unless SWIOTLB_FORCE is set).
  */
-unsigned int max_segment;
+static unsigned int max_segment;
 
 /*
  * We need to save away the original address corresponding to a mapped entry