ioat: ring size variables need to be 32bit to avoid overflow

The alloc order can be up to 16 and 1 << 16 will over flow the 16bit
integer. Change the appropriate variables to 16bit to avoid overflow.

Reported-by: Jim Harris <james.r.harris@intel.com>
Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>