ntb: Correct modinfo usage statement for ntb_perf

The order parameters are powers of 2; adjust the usage information
to use correct mathematical representations.

Signed-off-by: Gary R Hook <gary.hook@amd.com>
Fixes: 8a7b6a77

 ("ntb: ntb perf tool")
Acked-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>