NTB: Fix typo in setting one translation register

In the code for Xeon devices in back-to-back mode with xeon_errata_workaround
disabled, the downstream device puts the wrong value in SNB_B2B_XLAT_OFFSETL
(SNB_MBAR01_DSD_ADDR vs. SNB_MBAR01_USD_ADDR).

This was spotted while reading code, since the typo has no practical effect,
at least for now: the low 32 bits of both constants are actually identical
anyway.  However, it's clearer and safer to use the right name.

Signed-off-by: Roland Dreier <roland@purestorage.com>
Signed-off-by: Jon Mason <jon.mason@intel.com>