ntb: fix SKX NTB config space size register offsets

The offsets for the SZ registers are wrong. Updated.

Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Reported-by: Sandeep Mann <sandeep@purestorage.com>
Tested-by: Zachary Ross <zacharyx.ross@intel.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>