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>