ioat: fix xor_idx_to_desc

For versions of the device that implement operation-types 0x87, 0x88
(IOAT_OP_XOR, IOAT_OP_XOR_VAL) this map determines whether a given
source is located in the base or extended descriptor.  Source addresses
6 through 8 require an extended descriptor, hence 0xe0, not 0xd0.  No
shipping hardware currently implements these operation types.

Reported-by: Evgueni Smogailov <evgueni.smogailov@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>