NTB: Correct Number of Scratch Pad Registers

commit 87034511 upstream.

The NTB Xeon hardware has 16 scratch pad registers and 16 back-to-back
scratch pad registers.  Correct the #define to represent this and update
the variable names to reflect their usage.

Signed-off-by: Jon Mason <jon.mason@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>